GIFT   OF 
_  Fhoebe  Si  -  Means fi. 


PETROGRAPHIC  METHODS 


PART  I 
THE  POLARIZING  MICROSCOPE 

PART  II 
ROCK  MINERALS 


Published  by  the 

McGraw-Hill   Boole  Company 


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McGraw  Publishing  Company  Hill  Publishing  Company 

Publishers  of  Books  for 

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PETROGRAPHIC    METHODS 

THE  AUTHORIZED  ENGLISH  TRANSLATION  OF 

PART  I 

Anleitung  zum  Gebrauch  des  Polarisationsmikroskops 
(Third  revised  edition) 

AND 
PART   II 

Die  Gesteinsbildenden  Mineralien 
(Second  revised  edition) 


BY 

DR.  ERNST  WEINSCHENK 

PROFESSOR  EXTRAORDINARIUS  OF  PETROGRAPHY 

IN  THE 
UNIVERSITY  OF  MUNICH,   GERMANY 


RENDERED  INTO  ENGLISH  BY 

ROBERT  WATSON  CLARK,  A.  B. 

INSTRUCTOR  IN  PETROGRAPHY  IN  THE   UNIVERSITY  OF  MICHIGAN 


McGRAW-HILL    BOOK   COMPANY 

239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 

1912 


COPYRIGHT,  1912 

BY  THE 

McGRAw-HiLL  BOOK  COMPANY 


Printed  and  Electrotyped 

by  The  Maple  Press 

York,  Pa. 


PREFACE 

Although  there  are  several  excellent  treatises  on  rock  minerals 
in  thin  section  there  is,  nevertheless,  a  demand  for  a  text  which 
sets  forth  all  the  methods  used  in  a  detailed  study  of  rocks,  in  a 
clear  and  concise  manner.  The  fact  that  Part  I  of  the  original 
German  text,  published  by  the  Herder  Publishing  Company, 
of  Freiburg,  in  Breisgau,  appeared  in  the  third  edition  in  1910 
and  Part  II  in  the  second  edition  in  1907,  seems  recommendation 
enough  for  presenting  it  to  the  English  reading  student.  As  it 
was  found  inexpedient  to  adhere  strictly  to  a  mere  translation 
of  the  German  an  attempt  has  been  made  rather  to  render  it 
freely  into  English. 

I  am  greatly  indebted  to  Professor  E.  H.  Kraus,  of  the  Univer- 
sity of  Michigan,  for  encouragement  in  this  effort,  for  very  help- 
ful suggestions  in  its  execution,  and  for  material  aid  in  reading 
the  manuscript  and  correcting  the  proof.  I  wish  to  express  here 
my  sincere  appreciation  for  his  interest  in  this  work.  Also  to 
Mr.  W.  F.  Hunt,  of  the  University  of  Michigan,  I  express  my 
hearty  thanks  for  aid  in  reading  the  manuscript  and  correcting 
proof.  . 

R.  W.  CLARK. 
MlNERALOGICAL  LABORATORY, 

UNIVERSITY  OF  MICHIGAN, 
January,  1912. 


CONTENTS 

PAGE 
PREFACE . v 

INTRODUCTION xiii 


PART  I 

CHAPTER  I 

THE  MICROSCOPE 1 

The  Simple  Microscope  or  Lens 1 

The  Compound  Microscope 4 

Polarizing  Apparatus 7 

The  Polarizing  Microscope 12 

Material  for  Observation    .  21 


ERRATA. 

Page    18  Caption  to  Fig.  32a  should  read  Lomb. 

Page    22  Line  14                            "  "  1/2  mm. 

Page    64  ®  "  t 

Page  132  Caption  to  Fig.  161  Heating  Apparatus. 

Page  184           "        "     "     183       "  "  Trachyte. 

Page  188           "        "     "     195       "  "  Pyroxene. 


Clark  Petrographic  Methods. 

.  30 

'                      .  30 

.uinerences  in  inaices  ol  Ketraction  under  the  Microscope           '.    .  33 

Determination  of  the  Index  of  Refraction,  Immersion  Method      .  36 

Determination  of  Form  and  Cleavage 41 

Measurement  of  Size  and  Thickness 45 

Inclusions 47 

Color 48 

Observations  in  Reflected  Light 49 

CHAPTER  IV 

OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT 51 

1.  Optical  Properties  of  Crystals 51 

vii 


viii  CONTENTS 

PAGE 

Single  and  Double  Refraction 51 

Double  Refraction  of  Light  in  Calcite 52 

Uniaxial  Crystals 55 

Biaxial  Crystals 58 

2.  Investigations  with  One  Nicol — the  Polarizer 60 

Surface  Color  and  Pleochroism 60 

The  Phenomenon  of  Pleochroism     .    . 62 

3.  Investigations  with  Crossed  Nicols 65 

Recognition  of  Double  Refraction 66 

Determination  of  the  Position  of  the  Vibration  Directions     .    .  67 

Strauroscopes 71 

Strength  of  the  Double  Refraction.     Interference  Colors   ...  73 

Modification  of  the  Interference  Colors 77 

Measurement  of  the  Double  Refraction  by  Means  of  Interference 

Colors 79 

Interference  Colors  in  Variously  Orientated  Cross  Sections    .    .  82 

Chromoscope  for  Interference  Colors 83 

Character  of  the  Double  Refraction 87 

Compensators 88 

Use  of  Compensators 93 

CHAPTER  V 

OBSEKVATIONS  IN  CONVERGENT  POLARIZED  LIGHT 95 

Direction  of  the  Rays  in  Convergent  Light 95 

Methods  of  Observation  in  Convergent  Polarized  Light 97 

Optically  Uniaxial  Crystals 99 

Crystals  with  Circular  Polarization • 104 

Character  of  the  Double  Refraction  of  Uniaxial  Crystals     ....  105 

Biaxial  Crystals 107 

Dispersion  of  the  Optic  Axes 109 

Measurement  of  the  Optic  Angle 112 

Character  of  the  Double  Refraction  of  Biaxial  Minerals 115 

CHAPTER  VI 

TWINS  AND  OPTICAL  ANOMALIES 120 

Twins 120 

Optical  Anomalies 123 

APPENDIX.     (ACCESSORY  APPARATUS) 125 

1.  Rotation  Apparatus 125 

Rotation    Apparatus    for    Observations    between    two    Plano- 
convex Lenses 126 

Rotation  Apparatus  for  Investigation  in  Liquids 128 

2.  Heating  Apparatus 129 

3.  Microphotographic  and  Projection  Apparatus 132 

Drawing  Apparatus 134 

SUMMARY  OF  METHODS  .  .    135 


CONTENTS  ix 

PART  II 

CHAPTER  VII 

PAGE 

PREPAKATION  OF  MATERIAL 143 

Investigation  of  Rock  Pow.der 143 

Preparation  of  Thin  Sections 144 

CHAPTER  VIII 

METHODS  OF  SEPARATION 148 

1.  Chemical  Methods  of  Separation 148 

2.  Physical  Methods  of  Separation 152 

Analyses  by  Washing 152 

Separation  According  to  Specific  Gravity 153 

Heavy  Organic  Liquids 156 

Heavy  Solutions 157 

Heavy  Molten  Liquids      159 

Magnetic  Separation 160 

Other  Methods  of  Separation 161 

CHAPTER  IX 

METHODS  OF  INVESTIGATION 162 

1.  Chemical  Methods  of  Investigation 162 

(a)  General  Reactions 162 

Reactions  with  Ftuosilicic  Acid 163 

Reactions  with  Certain  Elements 164 

(b)  Special  Reactions 172 

Staining  Methods 173 

Precipitates  on  Thin  Section 175 

Alteration  by  Calcination 176 

2.  Physical  Methods  of  Investigation 177 

Determination  of  Specific  Gravity 177 

Determination  of  Hardness,  Cleavage,  etc 179 

CHAPTER  X 

DEVELOPMENT  OF  ROCK  CONSTITUENTS *  .  181 

External  Form 181 

Twinning 184 

Aggregates 185 

Growth  and  Solution 188 

Cleavage,  Parting  and  Mechanical  Deformation 190 

Intergrowth  and  Inclusions 192 

Directions  for  the  Use  of  the  Descriptive  Section 199 

CHAPTER  XI 

DESCRIPTIVE  SECTION 204 

1.  Opaque  Minerals 204 

Pyrite 204 


CONTENTS 

PAGE 

Pyrrhotite 205 

Chalcopyrite 205 

Galena 205 

Metallic  Iron 206 

Magnetite 206 

Chromite 207 

Hematite 207 

Ilmenite 208 

Graphite :    .  209 

Carbonaceous  Matter 210 

2.  Isotropic  Minerals 212 

Perovskite 213 

Sphalerite 213 

Garnet  Group 214 

Spinel  Group 218 

Periclase 219 

Boracite 219 

Rock  Salt 219 

Leucite 219 

Glass 220 

Analcite 222 

Sodalite  Group 223 

Opal.    .' 224 

Fluorite 225 

3.  Uniaxial  Minerals 225 

Rutile 227 

Anatase 229 

Cassiterite 230 

Wurtzite 230 

Zircon 230 

Xenotime 231 

Corundum 232 

Vesuvianite 233 

Gehlenite  Group 233 

Tourmaline 234 

•  Apatite 236 

Rhombohedral  Carbonates 238 

Eudialyte 242 

Scapolite  Group 242 

Alunite 243 

Beryl 243 

Brucite 244 

Quartz,  Chalcedony,  Tridymite 244 

Nepheline 249 

Apophyllite 250 

Chabazite 250 

Cancrinite 250 

Hydronephelite 251 


CONTENTS  xi 

PAGE 

4.  Biaxial  Minerals 251 

Brookite 254 

Goethite 254 

Pseudobrookite 255 

Sulphur 255 

Baddeleyite 255 

Titanite ._.    .    .   255 

Lievrite 256 

Monazite 257 

Lavenite 257 

Chrysoberyl 257 

Epidote  Group 258 

Staurolite : 266 

Diaspore 267 

Cyanite 267 

Sapphirine 268 

Serendibite 269 

Prismatine 269 

Astrophyllite 269 

Brittle  Micas 269 

Margarite 272 

Olivine  Group 272 

Pyroxene  Group 277 

Lawsonite 286 

Amphibole  Group 287 

Dumortierite 295 

Axinite     . 295 

Rinkite 295 

Sillimanite 295 

Datolite 296 

Mosandrite 297 

Barite 297 

Andalusite 297 

Lazulite 298 

Carpholite 299 

Prehnite  .    . 299 

Celestite ' 299 

Aragonite 299 

Wollastonite  Group 299 

Humite  Group 300 

Topaz 301 

Anhydrite    .    . 301 

Mica  Group 302 

Chlorite  Group  and  Serpentine 310» 

Hydrargillite 316 

Talc 316 

Pyrophyllite 317 

Bertrandite.  .   317 


xii  CONTENTS 

PAGE 

Wagnerite 318 

Kaolin 318 

Nontronite 319 

Hydromagnesite 319 

Cordierite 319 

Wavellite 320 

Gypsum 321 

Feldspar  Group .321 

|     Biaxial  Zeolites 341 

TABLES 343 

INDEX    .                                                                                                            .  385 


Introduction 

The  investigation  of  inorganic  substances  does  not  entail  a 
study  of  an  almost  inexhaustible  number  of  forms  as  is  the  case 
in  the  organic  world  where  the  structure  is  often  so  detailed  that 
even  the  strongest  objectives  of  a  modern  microscope  cannot 
resolve  them.  Inorganic  bodies  occur  in  a  comparatively  large 
number  of  different  forms  and  strong  magnification  is  rarely 
needed  in  the  investigation  of  them,  because  their  structure  is 
seldom  very  minute. 

The  methods  of  microscopic  investigation  employed  in  the 
study  of  organic  nature  are  not  applicable  in  inorganic  research 
because  of  the  infinite  number  of  chemical  compounds  already 
known  and  the  large  additions  that  are  being  made  daily.  If 
the  microscope  were  merely  an  instrument  for  magnification,  as 
it  is  with  zoologists,  botanists,  and  medical  investigators,  neither 
the  chemist  nor  the  mineralogist  would  be  warranted  in  under- 
taking microscopic  studies. 

Nevertheless,  the  modern  microscope  is  very  useful  in  the 
investigation  of  inorganic  substances.  It  is  a  sort  of  optical 
universal  apparatus  which  reveals  not  only  the  outer  form,  but 
also  the  inner  structure  of  a  substance.  It  leads  to  results 
rapidly  and  easily,  which  if  obtained  by  other  methods  would 
involve  much  time  and  labor.  The  introduction  of  the  micro- 
scope into  chemical  and  mineralogical  laboratories  has  been 
made  possible  by  numerous  improvements  devised  in  the  last 
thirty  years  and  by  its  transformation  into  a  polarizing  instru- 
ment. It  is  not  yet  used  as  much  as  it  might  be  or  as  extensively 
as  its  adaptability  warrants.  The  splendid  aid  such  a  micro- 
scope affords,  is  too  little  appreciated,  especially  in  synthetic 
and  analytical  chemistry.  In  many  instances  there  is  no  other 
way  of  determining  the  characteristics  of  a  substance  so  rapidly 
and  positively,  making  lengthy  tests  often  unnecessary. 

Microscopic  methods  are  especially  serviceable  in  the  investi- 
gation of  rocks.  Petrography,  the  science  of  rocks,  owes  the 
great  progress  it  has  made  in  the  last  third  of  the  nineteenth 
century  to  microscopic  investigations.  These  methods,  so 

xiii 


xiv  INTRODUCTION 

fruitful  to  the  petrographer,  have  only  recently  been  applied  to 
chemical  investigations,  but  their  general  introduction  can  be 
accomplished  only  with  considerable  effort.  However,  micro- 
chemical  investigations  based  upon  modern  microscopic  technic 
have  yielded  results  that  are  extremely  promising. 

Anyone,  who  has  had  opportunity  to  study  the  methods  of 
microscopic  analysis  thoroughly,  must  confess  that,  with  a  little 
practice,  they  are  very  useful  in  recognizing  rapidly,  easily,  and 
positively  the  composition  of  a  substance  even  in  very  small 
quantities.  Nevertheless,  the  optical  methods  found  at  first  only 
a  very  limited  use  in  this  promising  field.  Synthetic  chemistry 
has  made  use  of  microscopic  methods  still  less,  although  they 
would  be  very  helpful  to  organic  chemists  in  particular  on 
account  of  the  large  number  of  compounds.  The  unnecessary 
loss  of  time  which  would  be  saved  if  organic  chemists  had  a 
means  of  identifying  their  compounds  both  rapidly  and  accu- 
rately, no  one  can  estimate.  If  it  were  only  known  how 
quickly  all  the  optical  properties  of  a  crystalline  precipitate  can 
be  determined  after  a  little  practice,  it  would  be  found  justifiable 
to  recommend  most  urgently  that  chemists  become  skilled  in 
microscopic  technic.  The  usual  statements  made  in  the  litera- 
ture concerning  such  precipitates  generally  take  into  considera- 
tion only  the  simplest  morphological  properties  and  these  are 
frequently  the  least  constant  characteristics  of  a  substance. 

Chemists  have  adopted  petrographic  methods  in  spite  of  the 
fact  that  their  materials  are  quite  different.  In  a  way,  a  petro- 
grapher is  at  a  disadvantage  because,  in  the  investigation  of 
slides,  he  has  to  deal  with  sections  orientated  at  random  while  in 
the  crystalline  powders,  which  a  chemist  studies,  the  various 
crystals  assume  a  common  characteristic  position  on  the  object 
glass  on  account  of  their  development.  It  is  much  easier  to 
determine  the  crystal  system  in  the  latter  case,  which  is  not  easily 
done  in  many  instances  in  rock  slides.  On  the  other  hand, 
variously  orientated  sections  of  one  and  the  same  mineral  can 
be  studied  by  a  petrographer  and  he  is  able  to  determine  the 
optical  properties  of  a  substance  in  all  directions,  while  with 
isolated  crystals  this  is  often  impossible.  Some  knowledge  of 
physical  crystallography  is  an  absolute  prerequisite  for  investi- 
gations in  both  these  fields  and  its  successful  application  to 
microscopic  studies  requires  much  practice. 

The  first  demand  made  of  a  polarizing  microscope  is  that  its 


INTRODUCTION  xv 

combinations  of  lenses  shall  be  as  perfect  as  possible.  Mag- 
nification is  of  secondary  importance,  the  other  optical  adjust- 
ments being  the  most  important  part  of  the  instrument.  A 
large,  plane,  achromatic  field  combined  with  objectives  and  ocu- 
lars transmitting  as  much  light  as  possible  are  the  first  things 
demanded  of  a  microscope  by  a  mineralogist  as  well  as  by  a 
botanist,  zoologist,  or  medical  investigator.  The  lens  systems  are 
used  for  the  various  optical  methods,  which  have  been  developed 
in  microscopic  technic,  and  the  optical  properties,  as  well  as  the 
observations  of  the  image  of  the  object  investigated,  are  depen- 
dent upon  the  sensitiveness  of  the  lenses. 

The  mineral  constituents  of  rocks  can  be  recognized  by  the 
naked  eye  only  in  a  few  cases.  Macroscopic  examination  is 
never  sufficient  to  reveal  all  the  details  of  the  composition  of  a 
rock.  Texture  also,  which  is  of  great  importance  in  the  classi- 
fication of  many  rocks,  can  be  observed  by  the  unaided  eye  only 
when  it  is  comparatively  coarse.  Before  a  thorough  study  of 
the  composition  and  texture  of  rocks  is  undertaken,  the  student 
should  become  well  acquainted  with  the  means  by  which  such 
knowledge  may  be  obtained. 

Microscopic  examination  of  thin  sections  is  very  important. 
It  is  quite  necessary  for  the  student  to  become  as  well  acquainted 
with  microscopic  optical  methods  as  possible  even  though  only  a 
superficial  knowledge  of  petrography  is  desired,  because  an 
approximate  determination  of  rocks  is  frequently  impossible 
without  them.  On  the  other  hand,  to  think  that  the  macroscopic 
characteristics  of  a  rock  should  be  ignored  and  the  same  deter- 
mined only  in  thin  section  under  the  microscope,  is  an  error  that 
must  be  avoided,  although  it  was  prevalent  during  the  earlier 
stages  of  the  development  of  petrography.  The  macroscopic 
appearance  of  a  rock  with  reference  to  texture  and  constituents 
furnishes  a  clue,  which  in  many  instances  simplifies  or  supple- 
ments the  microscopic  investigation.  For  this  reason  the 
general  exterior  appearance  of  a  rock  must  be  carefully  observed 
before  the  microscopic  investigation  is  begun.  Mineralogy  is 
the  science  that  teaches  one  to  recognize  the  minerals  by  their 
macroscopic  properties.  Petrographic  investigation  should  not 
be  undertaken  without  a  thorough  knowledge  of  mineralogy, 
although  the  list  of  rock-forming  minerals  of  frequent  occurrence 
is  very  small.  Mineralogy  is  the  indispensable  foundation  upon 
which  petrographical  investigation  is  based. 


xvi  INTRODUCTION 

Good  results  cannot  be  obtained  in  the  microscopic  study  of 
rocks  unless  the  student  has  a  comprehensive  knowledge  of  the 
optical  properties  of  crystals  and  has  had  much  practice  in  the 
application  of  microscopic  methods.  Familiarity  with  a  polariz- 
ing microscope  is  a  prerequisite  for  all  petrographic  work. 
Serious  errors  in  the  determination  of  rock  constituents  may  be 
avoided  by  such  knowledge.  Special  emphasis  must  be  laid 
upon  the  fact  that  simply  knowing  the  appearance  of  certain 
minerals  in  some  rocks,  or  in  a  large  number  of  rocks,  is  not 
sufficient  to  give  that  confidence  which  is  necessary  in  petro- 
graphic investigation.  As  the  appearance  of  one  and  the  same 
mineral  varies  greatly  in  different  rocks,  a  continual  source  of 
error  is  thus  introduced  and  only  general  confusion  results  if  the 
knowledge  of  minerals  is  purely  superficial  and  not  sufficiently 
based  on  microscopic  optical  methods. 

The  microscopic  optical  methods  necessary  for  petrographical 
investigation  are  discussed  thoroughly  in  the  first  part  of  this 
book.  A  knowledge ,  of  these  fundamental  principles  must 
precede  all  petrographic  work. 

A  positive  statement  concerning  the  detailed  composition  of  a 
rock  cannot  always  be  made  upon  the  results  of  microscopic 
investigations  alone.  A  thin  section  confines  the  examination  to 
an  extremely  limited  portion  of  the  rock.  In  general,  therefore, 
one  cannot  expect  to  ascertain  the  complete  composition  of  a 
rock  even  after  a  very  exhaustive  study  of  one  or  more  thin 
sections.  This  is  especially  true  of  certain  constituents  that  are 
only  sporadically  present,  but  still  are  of  considerable  importance 
for  the  rock  as  a  whole.  It  often  happens,  after  careful  investi- 
gation by  optical  methods,  of  certain  mineral  sections  in  a  slide, 
that  the  student  does  not  feel  sure  that  he  has  determined  the 
mineral  correctly.  An  attempt  is  made  to  supplement  these 
methods  as  much  as  possible  in  various  ways  and  this  may  be 
done  either  by  a  series  of  chemical  tests  on  the  slide  itself  or  by 
isolating  the  different  constituents  by  chemical  or  physical  means 
so  as  to  facilitate  studies  on  them.  We  distinguish,  therefore, 
between  methods  of  separation  and  methods  of  investigation,  and 
each  of  these  is  divided  into  a  chemical  and  a  physical  group. 
A  clear  conception  of  the  mineral  composition  of  a  rock,  upon 
which  a  safe  classification  is  possible,  can  be  obtained  only 
through  a  combination  of  all  these  methods. 

Finally  it  may  be  added  that  the  most  accurate  determination 


INTRODUCTION  xvii 

of  the  composition  of  a  rock,  obtainable  by  a  combination  of  all 
these  methods,  is  one  of  the  goals  of  petrography,  but  that  with 
it,  the  final  object  of  petrographical  research  has  by  no  means 
been  reached.  It  strives  to  explain  not  only  the  present  con- 
dition of  the  rocks,  but  also  the  origin  and  alteration  of  them. 
All  descriptions  of  rocks  should  be  full  enough  to  permit  of  the 
determination  of  their  geological  relations.  Petrographical 
Investigation  becomes  important  only  in  connection  with  geo- 
logical studies  and  in  this  respect  it  is  one  of  the  most  important 
chapters  in  the  whole  science  of  geology.  Without  its  aid  geology 
comes  to  some  conclusions  which  have  been  proved  in  many 
instances  to  be  erroneous.  The  purpose  of  this  book  is  to  make 
the  determination  of  the  rock-forming  minerals  possible  and 
space  cannot  be  devoted  to  the  consideration  of  these  other 
points.  Description  of  the  minerals  and  the  methods  of  studying 
them  is  the  sole  object  of  this  text. 


PART  I 

THE  POLARIZING  MICROSCOPE 


CHAPTER  I 


FIG.  1. — Real  Image. 


The  Microscope 

The  Simple  Microscope  or  Lens. — The  simplest  form  of  the 
microscope,  the  lens,  serves  to  shorten  the  focal  distance  of  the 
eye  and  to  increase  the  focal  angle  so  that  objects  which  are  too 
near  to  be  seen  distinctly  by  the  naked  eye  can  be  seen  by  the 
use  of  the  lens.  The  shortest  distance  that  a  normal  naked  eye 
can  see  distinctly  is  about  25 
cm.  If  the  distance  from  the 
eye  be  less,  the  image  becomes 
indistinct  and  it  requires  a 
system  of  lenses  to  make  it 
sharp.  Magnification  is,  there- 
fore, always  reduced  to  the 
focal  length  of  25  cm.  A  convex  lens  casts  a  real  image  SR, 
Fig.  1,  of  the  object  rs,  which  is  farther  from  the  lens  than  its 
focal  length.  The  image  is  inverted,  on  the  opposite  side  of  the 
lens  from  the  object,  and  is  beyond  the  focal  length  of  the  lens. 
It  usually  cannot  be  observed  by  the  eye  directly,  but  becomes 

visible  when  cast  upon  a 
screen  and  can  be  repro- 
duced directly  on  a  photo- 
graphic plate.  If  the  ob- 
ject is  less  than  the  focal 
distance  from  the  lens,  the 
rays  do  not  unite  in  a 
point  on  the  other  side  of 
the  lens,  Fig.  2,  but  the  image  RS  appears  in  a  normal  position 
on  the  same  side  as  the  object  rs,  and  is  designated  as  a  virtual 
image.  It  cannot  be  projected. 

The  faces  of  a  cor. vex  lens  are  portions  of  spherical  surfaces  and,  in  conse- 
quence of  this,  the  rays  passing  through  various  parts  of  the  lens  converge 
approximately  at  a  common  point  only  when  the  curvature  of  the  lens  is 

1 


FIG.  2. — Virtual  Image. 


FETROGRAPHIC  METHODS 


FIG.  3. — Spherical  Aberration. 


very  small.     If  the  faces  have  a  greater  curvature  the  rays  through  the  edge 

unite  at  another  point  than  that  at  which  the  rays  through  the  center  con- 
verge, and  we  see  the  phenomenon 
of  spherical  aberration,  Fig.  3.  This 
causes  convexity  of  the  image  and  in- 
distinctness in  various  portions  of  the 
field  of  vision,  which  is  very  annoy- 
ing and  fatiguing  to  the  eye.  The 
image  appears  distorted  because  all 
parts  of  the  field  are  not  magnified 
equally.  The  border  is  magnified 

more  than  the  center  so  that  a  cross-sectioned  object  like  Fig.  4  appears  as 

indicated  in  Fig.  5. 

Another  deficiency  of  a  simple  lens 

is  its  dispersion,  i.e.,  the  property  of 

bodies  to  refract  light  of  various  wave 

lengths  differently.      In  the  normal 

case,  colors  having  the  shortest  wave 

lengths  are  refracted  most,  i.e.,  violet 

rays   are    deviated   more    than    red. 

The    formula    for  such  dispersion  is 

v>  p.     It  thus  happens  that  the  ob- 
ject appears  to  have  a  colored  border 

because  the  various  colors  converge  at 

different  places,  Fig.  6.     This  phenomenon  is  called  chromatic  aberration. 


FIG.  4.  FIG.  5. 

FIGS.  4  and  5. — Cross-sectioned  Object 

Magnified 
Uniformly.  Ununiformly. 


FIG.  6. — Chromatic  Aberration. 


These  two  difficulties  can  be  reduced  if  only  those  rays  which 
L  'pass  through  the  center 

of  the  lens  are  used. 
This  was  accomplished 
in  the  older  types  of 
lenses  in  the  manner 
indicated  by  Figs.  7  and 
8,  the  former  being  the 
Brewster,  the  latter  the  Coddington  model.  The  outer  part  of 
the  lens  is  cut  off  in  each  case  by  grinding  out  a  portion.  The 
elongated  cylindrical  lens  devised 
by  Stanhope,  Fig.  9,  also  reduces 
this  deficiency.  On  it  the  face 
turned  toward  the  object  has  less 
curvature  than  the  other  face.  All 
of  these  models  have  the  disad- 
vantage that  their  focal  lengths 
are  very  short.  Fraunhofer  im- 
proved upon  them  by  combining  two  plano-convex  lenses  set 


FIG.  7. 
Brewster's  Lens. 


FIG.  8. 
Coddington's  Lens. 


THE  MICROSCOPE 


with   their   convex    faces    toward    each    other,   Fig.    10.      This 

model  has  been  varied  in  many  ways. 
Lenses    which   are   completely   or 

almost  entirely  corrected  for  spherical 

and  chromatic  aberration  are  said  to 

be  aplanatic  and  achromatic.     Such  a 

condition  is  obtained,  however,  only 

by  a  combination  of  different  kinds 

of  glass.     Thus,  flint  glass  has  nearly  twice  as  great  a  dispersion 

as  crown  glass.     A  system  of  lenses 
can  be  arranged  as  in  Fig.  11,  con- 
sisting   of    a    converging   lens   L  of 
crown  glass  and  a  double   concave 
lens  of  flint  glass  L'.     The  colors  are 
FIG.  11.— Correction  of  chromatic    dispersed  by  the  convex  lens,  but  are 
Aberration.  reunited  in  the  point  p  by  the  con- 

cave lens,  the  so-called  Bruecke  lens.     The  best  modern  lens  is 

one   composed   of   a  double  convex  lens   of 

crown  glass  L,  Fig.  12,  between  two  diverging 

meniscuses  of  flint   glass   F.      It   is    almost 

perfectly  aplanatic  and  achromatic  and  com- 
bines with  these  advantages  a  large  field,  a 

long    focal    length,    and    distinctness.      The 

entire  field,  which  is  very  large,  is  only  used 

when  the  lens  is  placed  very  close  to  the  eye. 


FIG.  12. — Steinheil 
Triplet. 


It  is  used  princi- 
pally in  the  investigation  of 
rocks  when  a  magnification 
of  six  to  twelve  is  desired. 

The  Verant  lens,  consisting  of  a 
combination    of    two    lenses,    is 
quite   serviceable  -for    a    smaller 
magnification  up  to  about  four. 
It    has    a    horn    eyepiece  which 
must  be  fitted  to  the  eye  and  this 
places  it  in  the  proper  position  to 
I   obtain    an    image    entirely   free 
]  from  distortion.     A  Zeiss  anastig- 
j  matic  lens  can  be  used  for  stronger 
'    magnification       up       to     about 
twenty-seven.      It  consists  of  a 
combination  of  four  lenses  and  in 
spite    of   its    comparatively    large 
magnification  has  a  wide  field  and  the  object  can  be  placed  quite  a  con- 


FIG.  13. — Lenstand. 
(After  Voigt  &  Hochgesang.) 


PETROGRAPHIC  METHODS 


siderable  distance  from  it.     The  lens  ought  to  be  held  firmly  in  a  stand  for 
that  purpose  while  the  eye  is  moved  slowly  over  it. 

It  is  best  to  fasten  the  lens  upon  a  stand,  especially  when  stronger  magni- 
fication is  used,  to  insure  the  necessary  stability  and  to  leave  the  hands  free 
for  manipulation.  Fig.  13,  page  3,  shows  such  a  stand  equipped  to  use 
polarized  light.  It  may  also  be  provided  with  a  rotating  stage. 

The  Compound  Microscope. — The  compound  microscope,  or 
simply  the  microscope,  is  distinguished  from  the  simple  micro- 
scope or  lens  by  a  combination  of  two  independent  systems  of 

lenses.  The  simplest  form 
of  such  a  microscope  consists 
of  two  double  convex  lenses 
as  shown  in  Fig.  14,  in  which 
the  path  of  the  rays  through 
the  microscope  is  shown 
diagrammatically.  One  of 
the  lenses,  ab  Fig.  14,  has  a 
shorter  focal  length  than  the 
other,  and  since  it  is  always 
next  to  the  object  it  is  called 
the  objective.  It  casts  a 
real  magnified  image  US  of 
the  object  rs.  This  image  is 
a  little  further  from  it  than 
the  focal  length  of  the  lens 
and  is  inverted.  It  is  pro- 
jected within  the  focal  length 
of  the  eyepiece  or  ocular  and 
is,  therefore,  again  magnified 
and  appears  as  a  virtual- 
image  at  S'R'.  This  has 
the  same  relative  position  as 
the  real  image,  i.e.,  it  is  in- 
verted with  respect  to  the 
object.  It  thus  appears  that 
the  objective  casts  a  real 
image  of  a  close  object  and  this  image  is  in  turn  observed  by 
means  of  the  ocular,  as  a  magnified  virtual  image.  Since  the 
real  image  is  inverted  with  respect  to  the  object  and  the  virtual 
image  not,  objects  observed  through  the  compound  microscope 
always  appear  reversed.  For  purposes  of  projection  such  as 


Fia.  14. — Passage  of  Rays  through  a  Compound 
Microscope. 


THE  MICROSCOPE  5 

is  common  in  photography,  the  ocular  is  generally  removed  and 
the  real  image  of  the  object  is  projected  upon  a  photographic 
plate  or  upon  a  screen. 

The  ocular  and  objective  are  placed  in  a  metallic  tube  to  exclude  all  dis- 
turbing outside  light  between  them  and,  as  shown  in  Fig.  14,  they  are  placed 
much  farther  apart  than  the  sum  of  their  focal  distances.  They  are  cor- 
rected for  a  fixed  distance,  but  nevertheless  the  ocular  must  be  so  arranged 
that  this  distance  can  be  altered  somewhat,  as  desired,  because  even  in  a 
correct  apochromatic  system,  object  glasses  of  various  thickness  may  require 
lengthening  or  shortening  of  the  tube. 

The  ocular  is  generally  set  in  the  upper  part  of  the  tube  and  (in  the  simpler 
instruments)  the  objective  is  screwed  into  the  lower  part  of  it.  However, 
if  the  objective  is  changed  frequently,  considerable  time  is  lost  in  mani- 
pulating, which  is  very  annoying.  To  avoid  this  the  whole  series  of 


FIG.  15. — Revolver.         FIG.  16. — Tongs.     Objective  Holders.       FIG.  17. — Groove. 

objectives  can  be  fastened  on  a  revolver  so  that  the  change  can  be  effected  by 
simply  rotating  it,  Fig.  15,  or  the  tube  can  be  fitted  with  an  objective  clamp, 
Fig.  16,  into  which  the  objective  can  be  slipped  by  means  of  a  ring  on  the 
upper  part  of  it,  so  arranged  that  it  is  always  approximately  centered. 
Finally,  a  guide  bar  can  be  constructed  on  the  objective,  which  fits  accurately 
into  a  groove  in  the  tube,  Fig.  17. 

A  more  thorough  discussion  of  the  construction  of  modern  objectives 
and  oculars  would  lead  us  too  far  and  they  are  generally  very  perfectly  con- 
structed by  the  more  reliable  firms.  It  is  evident  that  a  system  of  lenses  must 
be  even  more  achromatic  and  aplanatic  than  a  simple  lens.  At  best  a  system 
of  lenses  is  only  approximately  perfect  in  both  these  respects  and  the  image 
appears  quite  perceptibly  distorted  when  a  periscopic  ocular  is  employed  in 
order  to  use  the  whole  field  of  the  objective.  This  is  especially  true  with  low 
magnification.  Generally,  the  correction  for  chromatic  aberration  is 
parfect  only  for  a  part  of  the  spectrum,  so  that  a  faint  coloring  appears 
particularly  with  strong  objectives.  This  is  entirely  avoided  in  apochromatic 
lenses,  which  are  constructed  of  special  glasses.  In  them  there  is  an  equal 
difference  of  magnification  for  the  colors  in  all  zones  of  the  field  of  vision  and 
this  is  completely  counter-balanced  by  a  compensating  ocular,  constructed 
in  the  reverse  order. 


6 


PETROGRAPHIC  METHODS 


To  obtain  a  high  magnification  it  is  generally  advisable  to  use  a  strong 
objective  with  a  medium  to  weak  ocular,  because  the  amount  of  light  in 
the  image  is  entirely  dependent  upon  the  numerical  aperture  of  the  objec- 
tive. Abbe  defines  the  numerical  aperture,  or  simply  aperture,  as  the  pro- 
duct of  one-half  of  the  angle  of  aperture  of  the  lens  and  the  index  of  refrac- 
tion of  the  medium  between  the  objective  and  the  object.  Figs.  18  and  19 
show  that  the  angle  of  aperture  of  the  system  of  lenses,  i.  e.,  the  angle  of  the 
cone  of  rays  that  is  taken  in  by  the  objective  is  not  the  sole  criterion  for  the 
amount  of  light  from  a  given  point.  In  each  case  the  angle  of  the  cone  of 
light,  and  therefore  the  amount  of  light  transmitted  from  the  object,  is  the 
same.  Fig.  18  represents  a  type  of  a  dry  system  of  lenses  in  which  the  outer 
rays  of  the  cone  of  light,  when  extended,  fall  outside  of  the  objective  be- 
cause, when  they  pass  from  the  slide  into  the  air,  they  are  refracted  away 
from  the  normal  and  in  this  case  are  diverging  at  a  larger  angle  than  the  angle 
of  aperture  of  the  lens.  Thus  the  object  is  illuminated  by  a  smaller  cone 
of  light. 

If  a  liquid  is  placed  between  the  object  and  the  objective  the  latter  be- 
comes an  immersion  system,  Fig.  19.  The  liquid  should  have  an  index  of 
refraction  approximately  the  same  as  that  of  the  cover-glass  on  the  object 


FIG.  18. — Passage  of  Rays  in  a 
Dry  System. 


FIG.  19. — Passage  of  Rays  in 
an  Immersion  System. 


and  the  lower  lens  of  the  objective,  e.  g.,  oil.  The  light  upon  passing  into 
the  object  is  refracted,  as  in  the  other  case,  but  when  it  passes  through  the 
oil  into  the  objective  it  suffers  but  little  deviation.  Every  point  of  the 
object  is  illuminated  by  the  full  strength  of  the  cone  of  light  used. 

The  aperture  of  a  dry  objective  cannot  be  more  than  1  theoretically, 
because  n  =  1  and  the  maximum  of  sin  u  is  1 .  With  a  water  immersion  the 
theoretical  value  may  be  as  high  as  1.33,  when  half  of  the  angle  of  aperture 
is  90°,  but  this  is  impossible  in  practice.  With  oil  it  may  reach  1.5  and  with 
bromnaphthalene  it  may  be  over  1.6.  It  must  always  be  kept  in  mind  that 
the  lenses  of  the  objective,  the  cover-glass,  and  the  liquid  between  them  must 
have  the  same  indices  of  refraction.  The  increased  illumination  of  an  im- 
mersion objective  is  shown  by  the  fact  that  even  a  water  immersion  takes 
in  1.77  times  as  much  light  as  a  dry  system  with  the  same  angle  of  aperture. 

The  resolving  power  of  objectives  will  now  be  briefly  discussed.  The 
shortest  distance  that  can  be  distinguished  by  an  objective  is  represented  by 
the  quotient  of  A  divided  by  the  aperture  of  the  objective,  where  A.  is  the 
wave  length  of  the  light  employed.  Structures  of  0.00015  mm.  can  be  dis- 
tinguished by  a  monobromnaphthalene  immersion  making  use  of  the  violet 
rays  and  taking  their  impression  on  a  photographic  plate.  This  is  more 


THE  MICROSCOPE  7 

easily  accomplished  by  oblique  illumination  with  a  ray  of  light  or  with  a 
central  cone  of  light.  On  account  of  the  greater  wave  length  of  the  light 
in  the  illuminous  part  of  the  spectrum,  the  distances  must  be  about  twice 
as  great  to  make  the  object  distinct.  Finer  detail  can  be  obtained  by 
photographic  methods  than  by  direct  observation,  especially  when  only 
ultraviolet  rays  are  used.  The  objectives  must,  however,  be  especially 
corrected  for  these  rays.  Ultramicroscopy,  devoted  to  the  study  of  ex- 
tremely small  particles  of  approximately  molecular  dimensions,  is  only 
mentioned  here  for  the  sake  of  completeness. 

It  may  be  suggested  that  the  best  source  of  light  for  microscopic  investi- 
gation is  daylight  and  indeed  a  northern  sky  covered  with  thin  white  clouds. 
Direct  sunlight  as  a  source  of  illumination  is  extremely  disadvantageous  for 
the  eye  and  must  be  changed  into  diffused  light  by  the  use  of  a  screen. 
Only  two  of  the  artificial  sources  of  light  are  of  importance,  namely,  the 
electric  arc  and  the  Lassar  lamp.  The  latter  gives  a  blue  light  produced  by 
passing  it  through  a  color  filter. 

To  produce  monochromatic  light,  either  a  flame  colored  by  sodium, 
lithium  and  so  forth  or  Geissler  tubes  or  a  light  filter  may  be  used.  If  very 
intense  illumination  is  employed,  the  Abbe  spectro-polarizer  may  be  advan- 
tageously used.  It  is  inserted  in  place  of  the  regular  polarizer  and  gives 
illumination  from  a  small  part  of  the  spectrum. 

The  tube  is  fastened  to  the  stand  by  means  of  a  large  screw  for 
coarse  adjustment  and  a  micrometer  screw  for  fine  adjustment. 
It  is  movable  vertically  to  and  from  the  stage,  which  is  firmly 
fixed  on  the  stand.  The  illuminating  apparatus  is  seen  through 
a  hole  in  the  stage.  It  serves  as  a  collecting  system  for  the  light 
reflected  from  a  concave  mirror  below.  It  is  necessary  that  the 
aperture  of  the  illuminating  system  be  at  least  as  great  as  that 
of  the  objective  if  the  entire  aperture  of  the  latter  is  to  be  used. 
The  former,  however,  need  not  be  so  carefully  corrected  for 
spherical  and  chromatic  aberration.  The  Abbe  illuminating 
apparatus  is  of  the  most  perfect  construction  for,  when  it  is 
employed  as  an  immersion  system,  its  aperture  is  the  same  as  the 
strongest  objective  and  at  the  same  time  it  affords  .a  sufficiently 
large  illuminated  field,  when  used  with  the  weakest  objectives. 
A  condenser  is  used  in  the  simpler  microscopes,  but  it  must  be 
removed  when  using  low  power  objectives  because  of  the  small 
illuminated  field. 

Polarizing  Apparatus. — The  polarizing  microscope  differs 
from  an  ordinary  microscope  chiefly  in  having  an  attachment  for 
producing  plane  polarized  light.  Ordinary  light  vibrates  in  all 
planes  at  right  angles  to  the  direction  of  propagation  of  the  ray 
as  shown  in  Fig.  20.  Plane  polarized  light,  on  the  other  hand, 
vibrates  only  in  one  plane,  Fig.  21.  The  plane,  of  polarization 


8 


PETROGRAPHIC  METHODS 


PP,  Fig.  22,  of  a  ray  of  light  AB,  i.e.,  the  plane  in  which  the  poles 
lie,  is  always  perpendicular  to  the  plane  of  vibration  SS. 

In  using  transmitted  polarized  light,  the  direction  of  propaga- 
tion AB,  Fig.  22,  is  perpendicular  to  the  plane  of  the  object. 
The  light  vibrates  in  a  plane  at  right  angles  to  this  direction,  this 
plane  being  called  the  plane  of  vibration  SS',  Fig.  22,  while  its 
trace  on  the  plane  of  the  object  SS',  Fig.  21,  is  the  vibration 


FIG.  20.— Ray  of  Ordinary 
Light. 


FIG.  21.— Plane  of  Vibration  of  a 
Ray  of  Polarized  Light. 


direction  of  the  light.  The  velocity  of  the  light  is  dependent 
upon  the  rate  of  vibration,  or  in  other  words,  the  elasticity  of  the 
medium  under  investigation  in  a  direction  at  right  angles  to  the 
direction  of  propagation.  In  examining  a  slide  we  observe  the 
differences  in  the  optical  elasticity  in  different  directions,  which 
lie  in  the  plane  of  the  object  itself,  and  are  thus  perpendicular  to 
the  direction  of  propagation  of  the  light.  The  plane  of  polariza- 
tion PP',  Fig.  22,  is  perpendicular  to  the  plane  of  vibration  SS'. 


FIG.  22. — Ray  of  Polarized  Light  with  Plane  of  Vibration,  Plane  of  Polarization  and  Direc- 
tion of  Propagation. 

Ordinary  light  can  be  changed  into  partially  polarized  light  by 
reflection  or  refraction.  Polarization  by  reflection  is  most  com- 
plete when  the  light  strikes  the  reflecting  surface  at  a  certain 
angle,  which  is  dependent  upon  the  reflecting  substance.  This 
angle  of  .incidence  i  is  determined  by  the  formula  tan  i  =  n. 
This  angle  possesses  another  notable  property,  viz.,  that  the 
reflected  portion  of  the  ray  incident  at  that  angle  travels  at 
right  angles  to  the  refracted  portion.  It  is  called  the  angle  of 


THE  MICROSCOPE  9 

polarization,  and  for  ordinary  glass  is  about  57°.  Naturally 
it  varies  with  different  colors,  although  only  to  a  small  extent. 
By  repeated  reflection  at  this  angle  the  amount  of  light  may  be 
increased  and  for  this  reason  a  series  of  thin  glass  plates  is  often 
employed,  which  produce  almost  perfectly  polarized  light. 

A  ray  of  ordinary  light  L,  Fig.  23,  which  falls  upon  a  series  of 
glass  plates  at  the  polarizing  angle  is  partially  reflected  as  a  ray 
of  plane  polarized  light  P.  Its  plane  of  vibration  is  perpendicular 
to  the  plane  of  the  incident  light  LOB.  The  refracted  portion 
of  the  ray  P',  which  passes  through  the  plates  is  likewise  polarized, 


P" 

FIG.  23. — Polarization  by  Reflection  and  by  Refraction. 

but  not  so  perfectly.  Polarization  becomes  more  complete  the 
oftener  the  ray  is  refracted,  i.e.,  the  greater  the  number  of  plates. 
The  plane  of  vibration  of  the  refracted  polarized  ray  is  perpen- 
dicular to  that  of  the  reflected  ray  and  is  therefore  parallel  to  the 
plane  of  incidence.  Polarization  of  the  refracted  ray  is  the  more 
complete  the  smaller  the  angle  of  incidence.  Polarization  by 
refraction  often  becomes  quite  annoying,  especially  in  the  case 
of  very  strong  objectives,  for  example,  those  with  a  very  large 
angle  of  aperture,  because  in  them  peripheral  rays -of  light  often 
pass  through  at  oblique  angles  of  considerable  size. 

The  simplest  and  oldest  polarizing  instruments  are  based  upon 
the  use  of  a  reflecting  glass  plate.  Such  is  the  Noerremberg 
polariscope  or  orthoscope,  Fig.  24.  Light  from  a  strikes  the 
reflecting  plate  AB,  the  polarizer,  at  the  polarizing  angle  and  is 
reflected  as  plane  polarized  light  to  the  mirror  c,  and  from  it 
into  the  axis  of  the  instrument.  The  polarized  light  passes 
through  the  plate  AB  and  the  rotating  stage  above  it,  and  strikes 
the  blackened  mirror  S,  the  analyzer,  which  can  be  rotated  about 
a  vertical  axis.  When  S  is  parallel  or  antiparallel  to  AB,  it 


10  PETROGRAPHIC  METHODS 

reflects  the  light  polarized  by  the  latter,  but  when  in  a  crossed 
position  it  extinguishes  the  light  and  appears  dark. 

A  simple  polarizing  apparatus  can  be  prepared  from  double 
refracting  crystal  plates  in  which  the  absorption  of  light  in  one 
direction  is  much  greater  than  in  the  other,  for  example,  in  deeply 
colored  tourmaline.  Fig.  25  shows  tourmaline  tongs  of  this  sort. 
Each  arm  of  the  tongs  contains  a  plate  of  tourmaline  cut  parallel 
to  the  optic  axis  and  so  fastened  that  it  can  be  rotated.  The 
plates  are  so  thick  that  no  light  passes  through  in  the  direction 
of  greatest  absorption.  If  like  directions  in  the  two  plates  are 
parallel  light  is  transmitted,  but  in  crossed  positions  the  field  of 
vision  appears  dark. 


FIG.  24. — Orthoscope.  FIG.  25. — Tourmaline  Tongs. 

Polarization  of  light  by  reflection  or  refraction  is  by  no  means 
complete  enough  for  finer  investigations,  while  that  produced  by 
absorption  gives  polarized  light  which  is  deeply  colored.  For 
this  reason  such  apparatus  is  used  only  in  the  simplest  and 
cheapest  instruments,  while  in  a  good  polarizing  microscope,  nicol 
prisms  or  similar  devices  are  employed.  These  completely 
polarize  the  light  and  produce  colorless  illumination  with  but 
little  loss. 

The  construction  of  nicol  prisms,  or  simply  nicols,  depends  upon 
the  ability  of  double  refracting  crystals  to  decompose  a  ray  of 
ordinary  light  into  two  rays  of  polarized  light  vibrating  at  right 
angles  to  each  other.  These  rays  are  refracted  differently. 
The  original  form  of  a  nicol,  Fig.  26,  was  an  elongated  cleavage 
piece  of  calcite,  the  ends  of  which  were  cut  so  as  to  make  an  angle 
of  68°  instead  of  71°  with  the  long  edges.  Then  the  calcite  is 


THE  MICROSCOPE 


11 


cut  through  at  right  angles  to  the  artificial  faces  and  the  two 
halves  are  cemented  together  in  the  same  position  with  Canada 
balsam.  The  two  polarized  rays  vibrating  at  right  angles  to  each 
other  pass  through  the  lower  half  of  the  prism  as  far  as  the 
layer  of  Canada  balsam,  but  the  ordinary  ray  (o)  is  refracted 
more  than  the  extraordinary  ray  (e).  In  the  direction  in  which 
the  ordinary  ray  vibrates,  calcite  is  a  much  denser  medium  for 
light  than  Canada  balsam,  and  since  this  ray  e 

impinges  upon  the  layer  of  balsam  with  a  suffi- 
ciently high  angle  of  incidence  it  is  totally  reflected 
and  eliminated.  The  other  ray  (e)  is  refracted 
about  equally  in  calcite  and  Canada  balsam  and 
since  it  impinges  upon  the  balsam  with  a  smaller 
angle  of  incidence,  it  passes  through  the  combina- 
tion with  only  slight  refraction.  Thus  a  nicol  prism 
produces  light  polarized  in  one  plane,  the  plane  of 
vibration  being  that  of  the  extraordinary  ray  and  a 
principal  section  of  the  calcite.  The  vibrations  of 
the  ray  are  parallel  to  the  short  diagonal  of  a 
cleavage  piece  of  calcite. 

This  original  form  was  afterward  altered  and  modified 
in  many  ways.  One  modification  which  has  been  used 
quite  extensively  was  suggested  by  Hartnack  and  Praz- 
mowsky.  Aside  from  having  the  end  faces  perpendicular 
to  the  others  they  used  linseed  oil  instead  of  Canada  balsam 
as  the  cement.  This  allows  the  calcite  to  be  used  more 
advantageously.  Ahrens  produced  a  double  prism  of 
similar  construction  but  much  shorter,  and  it  is  character- 
ized by  an  especially  large  angle  of  aperture. 

Prisms  of  sodium  saltpeter  have  recently  been  recommended  extensively. 
On  account  of  its  optical  properties,  it  is  more  adaptable  for  such  apparatus 
than  calcite.  However,  its  application  has  been  limited  on  account  of  its 
hygroscopic  nature.  Combinations  of  glass  with  calcite  or  saltpeter  have 
been  used  with  great  success.  The  upper  portion  of  the  prism  may  be  con- 
structed of  glass  having  an  index  of  refraction  and  dispersion  as  nearly  as 
possible  identical  with  that  of  the  extraordinary  ray  in  calcite,  or  only  a  thin 
cleavage  piece  of  the  double  refracting  material  may  be  cemented  between 
two  glass  wedges. 

More  completely  polarized  light  can  be  obtained  with  such  a  combination 
than  with  a  simple  calcite  prism  in  which  the  light  reflected  from  the  edges 
is  always  a  disturbing  element.  Since  the  indices  of  refraction  and  particu- 
larly the  dispersion  of  the  glass  and  the  extraordinary  ray  in  calcite  are  not 
exactly  the  same,  such  combinations  can  only  be  used  as  polarizers  and  not 
as  analyzers  because  they  are  not  entirely  achromatic.  The  name  "nicol" 
has  been  retained  for  all  these  constructions. 


FIG.  26. 

Construction 

of  a  Nicol  Prism. 


12 


PETROGRAPHIC  METHODS 


The  Polarizing  Microscope. — Fig.  27  shows  a  cut  of  one  of  the 
simpler  polarizing  microscopes  made  by  W.  and  H.  Seibert.  A 
cross  section  of  this  instrument  is  shown  in  Fig.  28.  The  illumi- 
nation by  a  mirror,  as  well  as  the  ocular,  objective,  and  the 


FIG.  27.— Polarizing  Microscope  by  W.  &  H.  Seibert  in  Wetzlar. 


movement  of  the  tube  by  a  coarse  and  a  micrometer  screw  are 
the  same  as  in  an  ordinary  microscope.  This  instrument  differs, 
however,  from  the  ordinary  model  in  many  ways.  Cross  hairs, 
F,  Fig.  28,  consisting  of  two  fine  threads  of  spider's  silk  stretched 
exactly  at  right  angles  to  each  other,  are  placed  in  the  inner  focus 


THE  MICROSCOPE 


13 


of  the  Huygen's  ocular,  which  is  arranged  to  fit  into  the  upper 
end  of  the  tube. 

The  part  of  the  tube  carrying  the  ocular  can  be  drawn  out  and 
contains  a  slit  in  which  a  Bertrand-Amici  lens  can  be  placed,  S, 
Fig.  28.  The  drum  m  on  the  micrometer  screw  for  fine  focusing 
of  the  tube  is  graduated,  es- 
pecially on  the  larger  instru- 
ments, so  that  the  vertical 
movement  of  the  objective, 
can  be  measured  accurately. 

The  screw  k  for  coarse  ad- 
justment of  the  tube  should 
be  placed  quite  high  so  that 
the  objective  can  be  moved 
vertically  within  wide  limits, 
thereby  allowing  the  use  of 
high  apparatus  on  the  stage, 
for  example,  the  universal 
rotating  stage.  The  arm  A, 
of  the  stand  which  holds  the 
tube  must  be  curved  as  much 
as  possible  so  that  it  will  not 
prevent  the  rotation  of  the 
stage  with  such  accessory  ap- 
paratus on  it. 

A  nicol  prism,  the  analyzer, 
is  generally  placed  within  the 
tube  so  that  it  can  be  with- 
drawn horizontally.  Under 
certain  conditions  it  is  ad- 
vantageous to  rotate  the 
nicol  through  90°  by  means 
of  /,  a  circular  scale  indicat- 
ing the  angle  of  rotation.  In 
some  cases  the  nicol  is  fixed  and  then  another  nicol  W,  placed 
over  the  ocular,  is  used  when  rotation  is  necessary.  The  ocular 
is  then  supplied  with  a  graduated  ring  R  so  that  the  amount  of 
rotation  can  be  read.  The  latter  form  of  analyzer  is  found  only 
in  the  older  models. 

The  clamp  Z,  Fig.  27,  on  the  lower  end  of  the  tube  holds  the 
objectives  firmly.     A  slit  c  into  which  the  compensators,  placed 


FIG.  28.— Cross  Section  through  the  Micro- 
scope in  Fig.  27. 


14 


PETROGRAPHTC  METHODS 


between  two  small  glass  plates,  can  be  inserted,  is  made  in  the 
tube  above  the  clamp.  The  objective  holder  is  fitted  with  two 
micrometer  screws,  v  and  v' ,  at  right  angles  to  each  other  with 
springs  opposite  each,  so  that  the  objective  can  be  moved  slightly 
in  a  horizontal  direction.  After  each  change  of  objectives  the 

axis  of  the  latter  can  be 
centered  exactly  with  the 
axis  of  rotation  of  the  stage 
by  means  of  these  screws. 
They  are  therefore  called 
the  centering  screws. 

On  other  models  the 
centering  is  effected  on  the 
stage,  i.e.,  instead  of  ad- 
justing the  axis  of  the  ob- 
jective with  the  axis  of 
rotation  of  the  stage,  the 
center  of  the  stage  itself 
is  shifted  by  a  similar  pair 
of  centering  screws.  Cen- 
tering on  the  stage  is  not 
so  advantageous,  for  it 
lessens  the  stability  of  the 
stage,  which  is  of  consider- 
able importance,  especially 
with  high  magnifications. 

Frequent        centering 
•Hi          causes   much   time    to    be 
^aSSBiiPyflP  wasted,  especially  with  the 

older  instruments  on  which 
the  objectives  are  always 
screwed  in.  This  is  very 
annoying,  however,  so 
some  instruments  have  been  constructed  which  make  centering 
unnecessary.  Thus  in  the  French  model,  Fig.  29,  devised  by 
Nachet  (Paris) ,  the  objective  and  the  stage  are  connected  by  a 
strong  arm  and  can  be  rotated  simultaneously,  thus  avoiding 
all  eccentric  rotation.  This  can  also  be  accomplished  on  the 
microscope  shown  in  Fig.  30,  which  was  first  constructed  in 
Geneva.  The  objective  holder  TM  with  the  micrometer  screw 
M  is  fastened  onto  the  stage  and  rotates  with  it.  Especially 


FIG.  29. — Polarizing  Microscope 
by  A.  Nachet  in  Paris. 


THE  MICROSCOPE 


15 


beginners  are  at  a  disadvantage  in  using  these  models  for  the 
head  of  the  micrometer  s.crew  changes  its  position  with  rotation 
of  the  stage,  and  when  it  is  necessary  to  use  the  screw  there  is 
a  tendency  to  remove  the  eye  from  the  ocular. 


Ok 


FIG.  30. — Polarizing  Microscope  by  C.  Reichert  in  Vienna. 

This  defect  was  avoided  by  a  model  first  made  in  England, 
Fig.  31  being  a  similar  instrument  devised  by  Voigt  and  Hoch- 
gesang.  Here  the  two  nicols  can  be  rotated  simultaneously,  the 
rotation  of  one  being  transmitted  to  the  other  by  means  of  a 
vertical  rack  and  pinion.  The  object  and  stage  do  not  move. 


16 


PETROGRAPHIC  METHODS 


Models  of  this  sort  are  strongly  recommended  for  work  with  the 
rotation  apparatus,  to  be  described  later,  and  are  made  in  various 
designs. 

By  improving  this  model,  Voigt  and  Hochgesang  have  recently 
produced  a  new  instrument  called  a  polarizing  microscope-poly- 
meter,  Fig.  32.  S  is  the  coarse  focusing  screw  of  the  microscope 
which  is  constructed  particularly  wide,  and  T  is  a  lateral  micro- 


FIG.  31. — Polarizing  Microscope  with  Rotating  Nicols. 
in  Gottingen.) 


(Voigt  &  Hochgesang 


meter  screw.  This  is  a  new  feature  and  is  entirely  independent 
of  the  rest  of  the  instrument.  The  analyzer  can  be  rotated  with 
the  circle  R  alone  and  likewise  the  tube  with  the  ocular  and  cross 
hairs  can  be  rotated  on  the  circle  K2,  while  the  objective  remains 
stationary.  The  two  nicols  can1  be  rotated  simultaneously  by 


THE  MICROSCOPE 


17 


shifting  F.  Thus  this  new  model  is  characterized  by  great 
stability  and  convenience,  together  with  a  wide  range  of  use- 
fulness. 


FIG.  32. — Polarizing  Microscope  Polymeter.     (Voigt  &  Hochgesang  in  Gottingen.) 

The  stage  T,  Figs.  27  and  28,  can  be  rotated  about  a  vertical  axis  and  is 
graduated  so  that  the  amount  of  rotation  can  be  noted.     A  vernier  is  often 
placed  on  the  scale,  but  it  is  generally  of  little  value  because  the  accuracy  of 
2 


18 


PETROGRAPHIC  METHODS 


microscopic  measurements  is  much  too  small  to  permit  closer  reading  than 
i°,  an  amount  that  is  easily  estimated.  The  stage  is  also  equipped  with  a 
series  of  holes  for  attaching  accessory  apparatus.  Of  these  the  object  clamp 
is  very  important  and  is  used  to  hold  the  object  in  place  particularly  when 
the  stage  is  inclined.  The  form  shown  in  Fig.  33  is  the  most  satisfactory 


FIG.  32a. — Polarizing  Microscope.      Bausch  &  Lamb. 

because  it  allows  a  definite  point  on  the  object  to  be  orientated  rapidly  in 
the  center  of  the  field  of  vision.  It  is  also  useful  when  the  stage  can  be  fixed 
in  any  position  by  means  of  a  set  screw. 

A  mechanical  stage  is  frequently  used  on  the  larger  instruments  for 
investigations  particularly  with  strong  objectives,  Fig    34.     By  means  of 


THE  MICROSCOPE 


19 


FIG.  33.— Object  Clamp. 


two  screws  at  right  angles  to  each  other  the  object  can  be  moved  laterally. 
With  such  an  apparatus  the  whole  slide  can  be  investigated  much  more 
evenly  because  one  is  independent  of  the  more  or  less  arbitrary  movement  of 
the  hand.  If  the  grooves  in  which  the  stage  slides  are  equipped  with  scale 
divisions,  the  stage  itself  may  be  used  to  locate  the  portions  of  the  slide 
investigated  by  simply  noting  the  position  of  the  scale  with  reference  to  a 
fixed  position  of  the  object  on  the  stage. 

It  is  better  though  to  indicate  the  position  of  a  mineral  in  the  slide  either 
by  a  free  hand  colored  ring  or  by  means  of  an  object  marker.  The  latter  is 
adjusted  in  place  of  the  objective  after  the 
mineral  has  been  accurately  centered.  This 
little  apparatus  may  have  an  eccentric  diamond 
point  that  can  be  regulated  and  set  in  a  holder 
with  a  spring  so  that  it  rests  upon  the  cover- 
glass  when  the  tube  is  lowered.  Then  the  stage 

with  the  slide  held  firmly  in  place  is  rotated  and  a  small  circle  is  scratched 
on  the  cover-glass  marking  the  proper  spot.  The  marker  may  also  have  the 
form  of  an  objective  similar  to  the  one  above,  but  on  the  lower  end  there  is 
a  round  opening  which  can  be  smeared  with  some  oil  color,  ink,  or  a  solution 
of  shellac.  This  leaves  a  small  circle  on  the  cover-glass  of  the  slide. 

Beneath  the  stage  on  the  models  shown  in  Figs.  27  and  28 
there  is  a  plate  with  a  hole  in  the  center  through  which  a  tube 
can  be  moved  in  a  vertical  direction  by  means  of  a  lever.  The 
illuminating  apparatus  and  the  second  nicol,  the  polarizer  P, 
are  placed  in  the  tube.  It  is  very  desirable  to  be  able  to  remove 
the  nicol  and  the  illuminator  separately  from  the  tube,  i.e., 

each  piece  should  be 
fastened  in  a  separate 
holder.  The  illuminat- 
ing apparatus  consists 
of  a  lens  of  small  aper- 
ture upon  which  rests 
the  condenser  with  large 
aperture.  In  case  of 
observations  with  very 
weak  objectives  the  con- 
denser can  be  swung  out 
of  the  path  of  the  rays 

by  means  of  tongs  placed  within  the  rotating  stage.  When  the 
condenser  rests  upon  the  polarizer  and  the  tongs  are  freed  from 
it,  it  moves  in  a  vertical  direction  with  the  polarizer  and  other 
illuminating  apparatus.  An  iris  diaphragm  which  is  never  lack- 
ing on  zoological  and  medical  instruments,  is  placed  only  on  the 
larger  models  of  the  polarizing  microscope,  because  it  requires 


FIG.  34. — Mechanical  Stage  by  R.  Fuesz  in  Steglitz. 


20 


PETROGRAPHIC  METHODS 


rather  complicated  apparatus  and  the  same  effect  is  obtained  by 
the  vertical  movement  of  the  illuminating  apparatus. 

The  polarizer  can  be  rotated  and  there  is  either  a  series  of 
notches  on  the  holder  into  which  a  catch  on  the  polarizer  slips, 

or  there  is  a  graduated  scale  so 
that  it  can  be  placed  parallel  to 
the  cross  hairs  in  the  ocular  at  any 
time. 


FIG.  35. — Device  for  removing  the 
Condenser  by  A.  Nachet. 


Many  other  devices  have  been  sug- 
gested to  overcome  the  annoyance  of 
removing  the  condenser  by  hand.  The 
condensing  lens  may  be  placed  in  a 
simple  slide  in  the  stage  of  the  micro- 
scope, but  as  vertical  movement  is 
then  impossible,  there  can  be  no  grada- 
tion of  the  illumination  or  adjustment 

for  variation  of  thickness  of  the  object  glasses.  It  may  also  be  fastened  in 
a  fixed  clamp,  but  here  again  the  limit  of  vertical  movement  is  small,  which 
makes  an  iris  diaphragm  necessary.  The  lens  may  also  be  fastened  in  a 
slide  at  a  fixed  distance  above  the  illuminator  in  such  a  manner  that  it 
retains  the  vertical  movement,  but  can  be  drawn  out  laterally.  Finally 
Nachet  perfected  a  very  simple  device  which  does  not  interfere  with  the 
movement  of  the  illuminating 

la 


system.  It  consists  of  a  lateral 
screw  below  the  stage  by  means 
of  which  the  condenser  can  be 
rotated  into  position  over  the 
illuminating  apparatus  like  a 
visor,  Fig.  35. 

In  some  cases,  especially  with 
very  strong  objectives  or  with 
objects  that  transmit  but  little 
light,  it  is  advantageous  to  re- 
move the  polarizer  entirely,  be- 
cause it  absorbs  more  than  half 
the  light  at  one's  disposal.  For 
this  reason  it  had  better  be  set 
in  a  holder  separate  from  that  of 
the  rest  of  the  illuminating  appa- 
ratus, for  then  it  can  be  with- 
drawn from  the  tube  at  any 
time.  Orientation  of  the  nicol  by  hand  wastes  a  good  deal  of  time. 


FIG.  36. — Device  for  throwing  out  the  Polarizer 
by  W.  &  H.  Seibert. 


By 


means  of  a  device  made  by  W.  and  H.  Seibert,  Fig.  36,  the  polarizer  P  can  be 
moved  in  a  groove  by  the  knobs  K  and  K'  and  its -place  taken  by  a  hollow 
cylinder  with  an  iris  diaphragm  J.  Thus  the  polarizer  can  be  thrown  in  or 
out  without  changing  its  orientation  and  this  has  the  advantage  that  the 
instrument  can  be  used  for  the  most  exacting  investigations  in  organic 


THE  MICROSCOPE 


21 


microscopy  at  any  time.  The  illuminating  apparatus  in  this  device  can  be 
moved  together  with  the  other  in  a  vertical  direction  by  the  screw  S'.  The 
condenser  C  can  be  slipped  out  by  the  tongs  Z.  This  complicated  device 
can  also  be  replaced  by  the  simple  visor  arrangement  and  it  is  also  apparent 
that  an  Abbe  illuminating  apparatus  might  be  used. 

Fig.  32A  represents  one  of  the  latest  models  of  a  petrographic  microscope 
made  by  the  Bausch  and  Lomb  Optical  Company  in  Rochester,  N.  Y. 

Material  for  Observation. — The  largest  number  and  the  most  important 
microscopic  observations  are  made  in  transmitted  light.  It  is  therefore 


FIG.  37. — Cutting  and  Grinding  Machine  by  Voigt  &  Hochgesang. 


necessary  to  prepare  the  objects  for  investigation  so  that  they  will  be  suffi- 
ciently transparent  to  allow  the  inner  structure  to  be  studied  and  so  that 
there  will  be  no  overlapping  of  different  individuals,  and  at  any  one 
place  on  the  object  the  material  be  homogeneous.  In  organic  microscopic 
investigations  this  is  accomplished  to  a  large  extent  by  means  of  an  instru- 
ment called  a  microtome.  In  mineralogy  and  petrography  such  thin  prepa- 
rations can  rarely  be  used  on  account  of  the  difficulty  of  preparing  them. 
However  since  the  structure  of  the  formations  considered  here  are  by  no 
means  so  fine,  a  slice  of  a  rock  or  mineral  ground  to  a  thickness  of  from 


22 


PETROGRAPHIC  METHODS 


0.03  to  0.04  mm.  is  sufficient.     Slices  of  0.02  mm.  thickness  or  less  are  neces- 
sary only  in  rare  cases  for  detailed  study  of  objects  with  fine  structure. 

A  cutting  or  grinding  apparatus  is  used  for  preparing  thin  sections,  the 
larger  ones  being  driven  by  motor  or  foot  power  as  in  Fig.  37.  The  smaller 
machines  are  supplied  with  a  crank  to  be  turned  by  hand,  but  this  hinders 
the  operation  considerably.  The  specimen  is  fastened  on  to  the  holder  a  with 
shellac  cement  and,  by  means  of  the  weight  c,  is  pressed  against  the  cut- 
ting plate  d,  which  rotates  above  it.  The  best  cutting  plates  are  covered 
around  the  edge  with  diamond  dust  and  must  always  be  kept  moist  with  oil. 
Emery  or  carborundum  powder  and  water  can  also  be  used.  When  the 
specimen  has  been  cut  once  the  holder  a  is  moved  to  the  left  a  short  distance, 
depending  upon  the  desired  thickness  of  the  section  and  a  second  cut  is 
made.  The  necessary  thickness  naturally  varies  with  the  coherence  of  the 
rock,  but  must  be  in  the  neighborhood  of  1—2  mm.  Next,  one  surface  is 
finely  ground  with  emery  or  carborundum  dust  on  the  slightly  convex  disk 
m  which  rotates  horizontally  and  then  it  is  cemented  with  warmed  Canada 


FIG.  38. — Cross  Section  through  a  Slide. 

balsam  on  to  a  thick  glass.  Care  must  be  taken  that  the  cement  is  neither 
too  brittle  nor  too  soft  because  in  either  case  the  preparation  may  be  easily 
torn  loose  in  the  grinding.  It  is  best  to  evaporate  Canada  balsam  on  a 
water  bath  to  the  proper  consistency.  Its  viscosity  is  then  reduced  by 
heating  on  a  hot  plate.  Then  the  section  is  carefully  ground  down  to  the 
desired  thickness  by  using  successively  finer  grained  carborundum  or 
emery. 

The  slides  are  usually  cemented  on  to  an  object  glass  by  means  of  Canada 
balsam  and  are  then  covered  with  a  cover-glass,  using  the  same  cement. 
The  thickness  of  the  cover-glass  may  be  0.10-0.15  mm.,  but  not  more  on 
account  of  the  short  focal  lengths  of  the  stronger  objectives.  The  thickness 
of  the  cover-glass  is  of  little  importance  with  weak  objectives,  but  the 
stronger  ones  are  corrected  for  a  fixed  thickness,  and  if  other  thick- 
nesses are  used  the  length  of  the  tube  must  be  changed.  For  certain 
special  investigations,  principally  the  determination  of  the  index  of  re- 
fraction, the  preparation  is  left  uncovered.  When  the  grinding  has 
been  carefully  done  the  thickness  of  the  slide  may  be  quite  uniform  except 
that  it  may  decrease  toward  the  edges  as  shown  in  Fig.  38,  which  represents 
a  cross  section  through  the  edge  of  a  thin  section  magnified  about  50  times, 
a  is  the  cover-glass,  d  the  object  glass,  and  6  the  rock  slide  on  which  the  rough 
surfaces  can  be  distinctly  seen.  The  slide  itself  is  enveloped  on  both  sides 
with  Canada  balsam. 


THE  MICROSCOPE  23 

The  form  and  size  of  the  object  glass  plays  an  important  role,  while  the 
thickness  may  vary  between  limits,  which  depend  upon  the  focal  length  of 
the  illuminating  apparatus.  Square  object  glasses  about  32x32  mm.  are 
most  satisfactory  for  investigations  with  the  polarizing  microscope,  and  at 
the  present  time  these  are  almost  always  used  in  petrography.  If  a  longer 
form  is  preferred,  it  must  not  be  so  long  that,  when  placed  on  the  stage  of  a 
polarizing  microscope  and  rotated,  it  will  strike  some  part  of  the  instru- 
ment and  be  displaced. 

The  study  of  rock  powders  often  leads  to  good  results  for  rapid  orientation. 
This  was  the  earliest  method  of  microscopic  rock  analysis.  Thin  cleavage 
plates  of  minerals  having  a  good  cleavage,  often  give  very  characteristic 
optical  reactions  and  when  artificial  crystals  are  to  be  investigated  the  best 
results  are  obtained  in  many  cases  when  the  crystals  are  allowed  to  form  on 
the  object  glass  by  the  evaporation  of  a  drop  of  the  solution;  or  a  fine 
crystalline  powder  may  be  imbedded  in  Canada  balsam  or  some  other 
liquid. 

Reflected  light  is  used  comparatively  rarely  for  observations  with  a 
polarizing  microscope  except  for  opaque  objects,  minute  crystals,  and  etch 
figures.  Preparations  of  this  character  are  best  left  uncovered. 

It  may  be  mentioned  finally  that  the  methods  of  staining,  which  are  so 
important  in  organic  microscopy,  are  used  only,  in  exceptional  cases  in  the 
investigation  of  inorganic  bodies  and,  then  particularly  in  the  investigation  of 
loose  fibrous  or  scaly  structures,  but  even  then  they  are  of  less  value  than  in 
organic  preparations. 


CHAPTER  II 
The  Adjustment  of  a  Polarizing  Microscope 

Before  a  microscope  can  be  used  it  must  be  tested  to  see  that 
the  various  parts  of  the  instrument  perform  their  functions 
properly.  The  tests  consist  of: 

1.  Testing  the  system  of  lenses; 

2.  Centering  the  stage; 

3.  Adjusting  the  cross  hairs  and  the  nicol  prisms. 

These  operations  are  called  "the  adjustment  of  the  instrument." 

i.  Testing  the  Lenses 

Aplanatic  and  Achromatic  Properties. — Although  accurate  correction  for 
spherical  and  chromatic  aberration  in  the  illuminating  lenses  plays  a  com- 
paratively small  role  and  need  only  be  considered  for  microphotography,  yet 
the  sharpness  of  the  image  and  the  clearness  of  the  observations  depend  upon 
the  most  perfect  aplanatic  and  achromatic  properties  of  the  lenses.  Con- 
vexity of  the  image  is  the  first  thing  that  appears  when  using  a  defective 
objective.  It  may  be  recognized  by  inability  to  make  the  center  and  the 
edge  of  the  image  of  equal,  sharpness.  The  haziness  of  the  image  in  differ- 
ent parts  of  the  field  makes  work  with  such  objectives  extremely  fatiguing  to 
the  eye.  It  is  encountered  in  fairly  well  constructed  weak  objectives  only 
with  a  periscopic  ocular,  which  uses  the  entire  field  of  the  objective,  but  in 
stronger  lens  systems  it  may  affect  and  fatigue  the  eye  with  ordinary  oculars. 
Along  with  the  haziness,  there  is  a  distortion  of  the  image  due  to  the  fact 
that  different  zones  in  the  field  are  magnified  differently.  A  cross-sectioned 
micrometer  is  used  to  recognize  this  imperfection,  Figs.  4  and  5,  page  2. 
Chromatic  aberration  is  likewise  troublesome.  It  occurs  with  stronger 
magnification  and  is  not  entirely  corrected  in  the  best  achromatic  lenses. 
Minute  opaque  bodies  appear  colored,  especially  if  the  reflecting  mirror  is 
placed  obliquely  or  if  the  light  from  one  side  is  shut  off  by  a  screen.  If  the 
illumination  is  principally  from  the  left,  an  object  will  appear  reddish  on 
the  left  and  violet  on  the  right  if  the  chromatic  correction  has  been  insuffi- 
cient, and  if  too  great,  the  colors  are  reversed.  The  only  lenses  that  are 
perfectly  corrected  for  chromatic  aberration  are  the  apochromatic  lenses, 
but  even  with  these  a  compensating  ocular  must  be  used. 

The  system  of  lenses  may  be  tested  for  perfect  achromatic  correction  by 
the  Abbe  test  plate.  This  is  a  silvered  glass  plate  on  which  series  of  parallel 
lines  are  etched  at  different  microscopic  distances  from  each  other.  An 
apochromatic  system  makes  such  a  series  appear  perfectly  clear  and  with- 
out colored  borders. 

24 


ADJUSTMENT  OF  A  POLARIZING  MICROSCOPE  25 

Light   Intensity,  Magnification,  and   Resolving   Power. — The 

amount  of  light  which  passes  through  an  objective  is  dependent 
upon  the  aperture,  if  properly  constructed.  It  is,  however, 
influenced  quite  appreciably  by  small  errors  in  the  construction 
of  the  lens  system  so  that  there  may  be  an  appreciable  variation 
in  the  amount  of  light  in  different  systems  with  the  same  aper- 
ture. Since  the  amount  of  light  is  also  dependent  upon  the 
magnification  demanded,  only  the  most  carefully  designed  and 
constructed  objectives  may  be  used  for  the  highest  magnification, 
less  perfect  ones  being  practically  useless.  There  is  also  serious 
objection  to  the  unlimited  increase  in  the  magnification  of  lens 
combinations,  which  in  many  cases,  seems  to  be  the  chief  object 
of  the  manufacturers.  On  the  one  hand  the  amount  of  light  is 
not  sufficient  for  clear  observations,  while  on  the  other  hand  the 
maximum  sensitiveness  of  the  objective  is  limited  by  the  aper- 
ture. Abbe  compiled  a  table  showing  the  total  magnification 
that  may  be  expected  from  a  microscope  with  objectives  with 
the  following  apertures: 

Aperture  0.1         0.2         0.3         0.6         0.951        1.20         1.302 

Magnification         53.0     106.0     159.0     317.0     501.0       635.0       688.0 

Some  idea  of  the  amount  of  light  passing  through  an  objective 
can  be  obtained  by  testing  it  with  a  weak  ocular  in  diffused  light 
produced  by  a  white  thinly  clouded  sky.  One  testing  of  this 
character  is  generally  quite  sufficient  for  ordinary  practice. 

The  magnification,  however,  must  be  measured  directly,  because  it  is  often 
necessary  in  work  with  the  microscope  to  give  numerical  values  of  the  sizes 
of  the  objects  observed.  A  table  showing  the  possible  magnifications  with 
the  various  objectives  and  oculars  generally  accompanies  each  instrument, 
but  it  is  important  to  check  it.  The  simplest  method  of  accomplishing 
this  is  to  focus  on  an  object  micrometer  with  the  lens  system  to  be  tested. 
One  millimeter  is  divided  into  a  hundred  parts  and  the  image  is  projected 
upon  a  sheet  of  paper  by  means  of  an  Abbe  sketching  device,  to  be  described 
later.  A  few  of  the  lines  are  then  sketched  on  thet  paper  and  the  distance 
between  them  is  measured.  It  must  be  remembered  that  the  amount  of 
magnification  varies  with  the  length  of  the  tube  and  the  measurement 
should  be  made  with  the  normal  length  to  which  the  objectives  are 
corrected. 

Granting  that  its  construction  is  perfect,  the  resolving  power  of  an  ob- 
jective is  a  function  of  the  aperture.  This  can  be  determined  theoretically 
with  an  Abbe  apertometer.  For  practical  ^purposes  a  test  object,  which 
generally  accompanies  a  microscope,  is  to  be  preferred,  especially  one  made 

1  Greatest  aperture  of  a  dry  system. 

2  Greatest  aperture  of  an  ordinary  oil  immersion. 


26  PETROGRAPHIC  METHODS 

of  diatoms,  the  fine  surface  structures  of  which  may  serve  as  a  standard  of 
the  resolving  power  of  the  objective. 

The  pleurosigma  angulalum  with  its  characteristic  fluting  in  three  direc- 
tions is  the  best  object  for  testing  the  strongest  dry  systems.  The  fine 
details  of  sketch  of  the  surirella  gemma,  especially  the  striations  running 
perpendicular  to  the  fine  cross  fluting,  serve  to  estimate  the  sensitiveness 
of  an  oil  immersion  up  to  an  aperture  of  1.30.  An  Abbe  test  plate  with 
its  various  systems  of  lines  can  be  used  for  the  same  purpose.  The  observa- 
tions are  more  perfect  when  the  object  is  illuminated  by  oblique  rays  of  light. 

Finally  false  light,  i.e.,  light  produced  by  reflection  of  any  sort,  must  be 
avoided  in  microscopic  observations,  although  it  is  much  less  annoying 
here  than  in  microphotography.  For  microphotographic  purposes  all 
metallic  parts,  especially  within  the  tube  of  the  microscope,  should  be 
blackened  to  prevent  reflection. 

Under  certain  conditions  such  reflection  may  be  caused  by  the  lenses  of 
the  ocular  itself,  so  that  it  seems  advisable  to  place  a  device  on  the  ocular  in 
some  cases,  which  corresponds  to  a  lengthening  of  the  holder  of  the  ocular 
lenses  and  cuts  off  the  light  falling  obliquely  on  the  upper  lens  of  the  ocular. 
In  other  instances,  especially  with  weak  objectives  having  a  long  focal 
length,  the  light  under  the  objective,  which  falls  on  the  slide  and  is  reflected 
by  it  into  the  tube,  causes  a  great  deal  of  annoyance.  Confusion  can  be 
avoided  by  shutting  out  the  light  with  the  hand  or  by  placing  a  black  screen 
around  the  instrument.  Such  devices  must  always  be  arranged  so  that 
they  do  not  interfere  with  the  movement  of  the  instrument. 

A  thin  cleavage  plate  of  mica,  observed  in  convergent  polarized  light  is 
the  best  device  to  determine  whether  the  angle  of  aperture  of  the  illuminat- 
ing system  is  sufficient  for  the  objectives  with  the  largest  numerical  aperture 
that  may  be  employed.  If  an  immersion  objective  is  used,  the  illuminating 
apparatus  must  also  be  used  as  an  immersion.  The  interference  figure 
will  be  equally  il  uminated  throughout  the  field,  if  the  objective  and  the 
illuminating  system  are  properly  adjusted.  If  the  angle  of  aperture  of  the 
illuminating  apparatus  is  not  sufficient,  the  edge  of  the  image  will  appear 
dark.  If  the  adjustment  is  not  proper  the  field  of  vision  is  unevenly  illu- 
minated or  a  sharply  outlined  portion  of  the  field,  corresponding  to  the 
cross  section  of  the  polarizer,  appears  light. 

Microscopes  made  by  the  best  firms  rarely  require  the  adjustments 
mentioned  above.  Centering  of  the  axis  of  rotation  of  the  stage  and  ad- 
justment of  the  vibration  directions  of  the  nicols,  on  the  other  hand,  are 
dependent  upon  numerous  contingencies  and  are  not  infrequently  altered 
during  work,  so  that  a  frequent  revision  of  them  is  strongly  advised.  The 
steps  necessary  will  therefore  be  described  more  thoroughly. 

2.  Centering  the  Stage 

The  center  of  rotation  of  the  stage  must  fall  as  nearly  as  pos- 
sible in  the  axis  of  the  objective,  i.e.,  the  microscope  must  be 
centered,  or  the  displacement,  which  an  object  undergoes  upon 
rotating  the  stage,  prevents  exact  observation  to  a  large  extent. 


ADJUSTMENT  OF  A  POLARIZING  MICROSCOPE  27 


Centering  is  naturally  unnecessary  in  those  microscopes  in  which 
the  object  rotates  simultaneously  with  the  objective,  Figs.  29 
and  30,  page  14,  or  the  nicol  prisms  rotate,  Figs.  31  and  32, 
page  16.  In  centering  other  models  a  small  speck  on  the 
object  is  brought  exactly  to  the  intersection  of  the  cross  hairs 
and  the  stage  is  rotated  through  360°,  while  the  displacement  of 
the  speck  is  constantly  observed.  The  speck  will  generally 
describe  a  circle  whose  center  x,  Figs.  39  and  40,  does  not  coincide 
with  the  intersection  of  the  cross  hairs  o.  The  centering  screw 


FIG.  39. 


FIG.  40. 


Centering  the  Stage. 


A  is  turned  until  the  point  x  has  passed  through  the  distance  xr, 
i.e.,  until  it  falls  upon  the  cross  hair  lying  transverse  to  this  screw. 
Then  with  the  screw  B  it  is  moved  through  the  distance  ro,  i.e., 
the  center  of  the  circle  x  appears  to  lie  at  the  intersection  of  the 
cross  hairs.  The  speck  first  selected  is  again  brought  to  the  center 
of  the  cross  hairs  and  is  tested  to  see  whether  it  changes  its 
position  upon  a  complete  rotation  of  the  stage.  Generally  there 
is  a  slight  movement  and  this  is  corrected  in  the  same  manner 
as  before  until  centering  is  perfectly  accomplished. 

In  general  each  change  of  objectives  introduces  a  small  eccentricity, 
although  in  perfectly  constructed  instruments  having  objective  tongs,  Fig. 
16,  page  5,  this  error  is  not  large  enough  to  cause  a  noticeable  interfer- 
ence with  the  work.  The  centering  screws  operate  at  an  angle  of  45°  to 
the  cross  hairs  instead  of  parallel  to  them  in  certain  instruments,  especially 
where  they  are  placed  on  the  stage.  The  instructions  given  above  must  be 
changed  then  according  to  the  diagram,  Fig.  40. 

The  line  of  sight  of  the  microscope  must  coincide  exactly  with  the  axis 
of  rotation  of  the  stage  and  this  cannot  be  obtained  by  simply  centering. 


28  PETROGRAPHIC  METHODS 

The  adjustment  of  the  instrument  for  this  is  checked  by  placing  a  signal 
such  as  a  dark  cross  on  the  lower  lens  of  the  objective  and  illuminating 
it  by  a  vertical  illuminator  from  above.  This  is  then  reflected  by  a  plane 
parallel  mirror  laid  upon  the  stage.  If  the  instrument  is  properly  adjusted 
the  image  from  the  mirror  must  coincide  with  the  cross  and  must  not  change 
its  position  upon  rotating  the  stage. 


3.  Adjustment  of  the  Cross  Hairs  and  the  Nicol  Prisms 

It  is  important  to  know  whether  the  vibration  direction  of  the 
polarizer  is  from  front  to  rear  or  from  right  to  left  when  it  has 
been  set  in  the  proper  position  indicated  on  it.  In  the  earlier 
nicols  with  rhombic  cross  section,  as  already  explained,  the  vibra- 
tion direction  is  parallel  to  the  short  diagonal,  but  in  the  various 
later  makes  it  must  be  determined  in  each  case.  A  thin  deeply 
colored  tourmaline  crystal,  which  shows  darker  color  and  stronger 
absorption  of  the  light 'when  its  principal  crystallographic  axis 
is  at  right  angles  to  the  vibration  direction  of  the  polarizer,  is 
used.  A  pile  of  glass  plates  or,  in  the  simplest  case,  a  reflecting 
surface  can  also  be  employed.  The  light  polarized  by  reflection 
vibrates  perpendicular  to  the  plane  of  incidence.  If  such  a 
reflecting  surface  inclined  at  the  polarizing  angle  is  observed 
through  a  nicol  prism,  it  will  appear  dark  as  soon  as  the  vibration 
direction  of  the  nicol  is  perpendicular  to  it,  i.e.,  parallel  to  the 
plane  of  incidence. 

The  adjustment  of  both  nicols  must  be  tested.  If  the  micro- 
scope is  to  be  used  for  any  kind  of  measurements  in  polarized 
light  the  vibration  directions  of  one  of  the  nicols  must  be  as  nearly 
as  possible  parallel  to  one  of  the  cross  hairs,  which  are  exactly  at 
right  angles  to  each  other.  This  is  best  accomplished  by  the  aid 
of  a  colorless  needle-like  crystal  imbedded  in  Canada  balsam.  The 
crystal  must  have  parallel  extinction  and  an  index  of  refraction 
as  near  that  of  the  Canada  balsam  as  possible.  The  reaction  is 
especially  distinct  when  the  crystal  shows  an  intense  interference 
color  of  a  low  order  between  crossed  nicols. 

Long  needle-like  crystals  of  quartz  about  0.1  to  0.15  mm.  thick 
are  particularly  useful  for  this  purpose.  Cleavage  pieces  of 
anhydrite  and  so  forth  are  a  little  less  adaptable,  but  can  be  more 
readily  obtained.  If  a  long  edge  of  the  crystal  imbedded  in 
Canada  balsam  is  placed  parallel  to  one  of  the  cross  hairs  and  the 
vibration  direction  of  the  polarizer  is  exactly  the  same,  light  will 
pass  through  the  crystal  unaltered.  Quartz,  which  has  an  index 


ADJUSTMENT  OF  A  POLARIZING  MICROSCOPE  29 

of  refraction  nearly  the  same  as  Canada  balsam,  is  entirely  invis- 
ible between  crossed  nicols  and  remains  invisible  upon  rotation  of 
the  analyzer  through  90°.  If,  however,  upon  rotating  the  ana- 
lyzer a  slight  illumination  or  coloration  of  the  quartz  takes  place, 
it  is  an  indication  that  the  vibration  direction  of  the  polarizer 
does  not  correspond  exactly  with  the  direction  of  the  cross  hair. 
This  is  corrected  by  rotating  the  polarizer  until  the  phenomenon 
can  no  longer  be  noticed.  The  analyzer  is  checked  in  the  same 
manner,  the  polarizer  being  marked  in  its  proper  position  and 
then  used  as  the  movable  nicol.  The  quartz  crystal  is  then  placed 
parallel  to  the  second  hair  and  the  test  is  again  made  to,  see  that 
no  coloration  takes  place.  When  such  is  the  case,  we  have  proof 
that  the  vibration  direction  of  one  of  the  nicols  is  parallel  to  one 
of  the  cross  hairs  and  further  that  the  two  hairs  are  exactly  at 
90°  to  each  other. 

In  many  instances  it  is  impossible  to  find  a  position  of  the  nicol  prism 
in  which  the  crystal  entirely  disappears  because  very  frequently  an  optical 
disturbance  is  produced  by  the  lenses.  Partial  polarization  by  refraction, 
especially  in  the  stronger  systems,  may  also  occur.  Further,  the  lenses  may 
acquire  considerable  double  refraction,  due  to  tension  in  the  metallic  holder 
caused  by  rapid  changes  of  temperature.  If  the  tension  in  the  latter  case 
is  not  too  great,  a  normal  condition  of  equilibrium  will  be  attained  after  a 
short  time.  However,  if  the  effect  on  polarized  light  is  quite  distinct,  such 
a  lens  system  will  give  rise  to  much  inconvenience  and  inaccuracy.  It  may 
be  noted  that  if  the  optical  disturbance  becomes  apparent  upon  inserting 
the  polarizer,  the  defect  belongs  to  it,  but  in  other  cases  to  the  objective. 
Since  nicol  prisms  are  generally  set  in  cork,  the  volume  of  which  changes 
as  it  gradually  dries  out  or  is  affected  by  changes  in  temperature  and  in  the 
humidity  of  the  air,  the  tests  for  the  orientation  of  the  nicol  prisms  must 
be  frequently  repeated.  This  is  especially  advisable  before  accurate 
measurement  of  the  vibration  directions  of  a  crystal  is  attempted. 


CHAPTER   III 
Observations  in  Ordinary  Light 

Ordinary  light  is  unpolarized.  The  observations  to  be  con- 
sidered next  can  be  carried  out  without  the  use  of  the  nicol  prisms. 
The  following  properties  can  be  studied  in  ordinary  light:  (1) 
index  of  refraction,  (2)  form  and  cleavage,  (3)  size  and  thickness 
of  the  object,  (4)  inclusions,  (5)  color,  and  (6)  appearances  in 
reflected  light. 

Methods  of  Determining  the  Index  of  Refraction. — The  first 
and  most  important  observation  that  we  are  able  to  make  with 
a  simple  microscope  is  that  of  refraction.  A  ray  of  light  which 
passes  obliquely  from  one  medium  to  another  is  deflected  from 
its  original  direction,  i.e.,  it  is  refracted  because  the  rate  of  trans- 
mission of  light  in  various  mediums  is  generally  different. 

Kin  i      v 
Refraction  may  be  expressed  by  the  law:         —  =  —  Fig.  41,  in 

sin  r      v 

which  i  is  the  angle  between  the  incident  ray  and  the  normal,  r, 
that  of  the  refracted  ray,  and  v  and  v'  are  the  respective  velocities 
of  light  in  the  two  mediums.  It  follows 
from  the  above  formula  that  when  a  ray  of 
"  light  passes  from  a  medium  with  greater 


velocity  v  to  one  with  smaller  velocity  v',  it 
is  refracted  toward  the  normal,  because  then 
r<i.      If   in   the   above   equation   the  first 
FIG  41  medium  is  air  in  which  the  velocity  of  light 

Refraction  of  Light.       may  be  assumed  to  be  equal  to  one,  v  =1, 
the  index  of  refraction  of  the   second  me- 
dium with  respect  to  air  may  then  be  expressed  by  the  formula, 

,  or  the  velocity  is  the  reciprocal  of  the  index. 
v'      sin  r 

If  light  passes  from  a  medium  with  smaller  velocity  v'  into  one 
with  greater  velocity  v,  the  conditions  are  reversed,  and  the 
angle  between  the  normal  and  the  refracted  ray  is  larger  than  for 
the  incident  ray.  The  ray  is  refracted  away  from  the  normal. 
For  a  certain  angle  of  incidence  i,  the  value  of  r  will  equal  90°, 
i.e.,  light  incident  at  this  or  a  greater  angle  does  not  pass  into  the 

30 


OBSERVATIONS  IN  ORDINARY  LIGHT  31 

second  medium,  but  is  totally  reflected.  The  angle  of  incidence 
for  which  r  =  90°  is  called  the  critical  angle.  The  greater  the 
difference  of  velocity  in  the  two  mediums,  the  smaller  the  critical 
angle,  e.g.,  for  air  and  glass  (n  =  1.5)  it  is  40°  45',  for  the  diamond 
(n  -2.4)  it  is  23°  45'. 

If  the  ray  falls  perpendicularly  upon  the  contact  of  the  two 
different  mediums  sin  i  =  0,  and  hence,  sin  r  =  0  as  well,  i.e., 
normal  incident  light  suffers  a  change  in  velocity,  but  it  is  not 
deflected. 

If  isolated  crystals  are  observed  under  the  microscope  only 
those  parts  will  appear  clear  and  transparent  which  are  bounded 
by  smooth  parallel  surfaces  that  lie  at  least  approximately  per- 
pendicular to  the  axis  of  the  microscope.  It  is  only  in  such  cases 
that  the  transmitted  rays  suffer  very  little  or  no  deviation.  If 
the  inclination  of  the  surfaces  is  considerable,  total  reflection 
takes  place  at  the  contact  between  the  crystal  and  air.  Such 
portions  then  appear  dark.  If  the  faces  of  a  crystal  are  uneven 
or  if  unpolished  plates,  such  as  thin  sections,  are  being  studied, 
they  appear  more  or  less  clouded  because  the  unevenness  of  the 
surface  allows  only  a  portion  of  the  light  to  pass  through  unre- 
fracted,  while  the  flank  surfaces  of  the  depressions  and  elevations 
cause  total  reflection  of  the  light  due  to  their  inclined  positions, 
Fig.  38,  page  22. 

If  a  slide  with  rough  surfaces  is  immersed  in  water  (n  =1.33) 
and  covered  with  a  cover-glass,  the  rough  appearance  will  entirely 
disappear,  but  the  slide  becomes  far  more  transparent  because 
now  the  difference  in  refraction  between  the  crystal  and  the 
medium  surrounding  it  is  less  and  the  critical  angle  becomes 
greater.  If  a  liquid  with  higher  index  is  used  the  rough  appear- 
ance will  disappear  more  the  more  nearly  the  index  of  the  liquid 
approximates  that  of  the  substance  in  question.  When  the 
indices  of  the  two  mediums  are  exactly  alike  the  section  will 
appear  as  though  polished  and  the  uneven  surfaces  perfectly 
even,  but  if  a  liquid  of  higher  index  is  employed,  the  unevenness 
of  the  surface  will  reappear.  The  outline  of  the  crystal,  which 
when  uncovered  shows  very  distinctly,  will  gradually  disappear 
as  the  index  of  refraction  of  the  surrounding  liquid  approaches 
that  of  the  crystal  and  is  entirely  invisible  when  the  two  indices 
are  alike,  provided  the  crystal  is  colorless.  With  more  strongly 
refracting  liquids  it  reappears. 

In  many  instances  the  best  method  for  observing  this  difference 


32 


PETROGRAPHIC  METHODS 


of  index  of  refraction  is  in  as  nearly  parallel  light  as  possible. 
If  the  rays  which  pass  through  a  preparation  are  quite  divergent, 
then  at  each  point  on  the  uneven  surface  light  impinges  from 
many  different  directions.  Therefore  a  portion  of  the  light  will 
pass  through  all  parts  of  the  preparation,  and  the  unequal 
illumination  and,  consequently,  the  rough  appearance  of  the 
surface,  show  much  less.  Since  the  index  of  refraction  is  one  of 
the  most  important  properties  in  microscopic  determinations,  it 
is  customary  not  to  polish  the  slide,  and  observations  are  begun 
in  approximately  parallel  light  so  that  this  difference  will  appear 
more  distinctly. 

The  illuminating  apparatus  of  the  microscope  produces  rays  of 
light  which  are  more  or  less  convergent  so  that,  for  the  investiga- 
tion of  the  index  of  refraction,  rays  as  nearly  parallel  as  possible 


FIG.  42.  FIG.  43. 

Cone  of  Light  narrowed  by  Iris  Diaphragm.     Sinking  Condenser. 

must  be  isolated  from  the  cone  of  light.  Narrowing  of  the  cone 
of  light  is  generally  accomplished  by  means  of  an  iris  diaphragm 
J,  Fig.  42,  placed  a  short  distance  below  the  illuminating  appa- 
ratus. This  cuts  out  those  rays  which  pass  through  the  outer 
edge  of  the  lens.  The  light  passing  through  the  middle  part  of 
the  lens  is  only  slightly  convergent,  as  shown  in  Fig.  42.  The 
effects  of  this  device,  which  is  generally  considered  to  be  indis- 
pensable, can  be  produced  even  better  by  sinking  the  whole 
illuminating  apparatus  as  shown  in  Fig.  43.  When  it  is  lowered 
in  its  holder  the  outer  part  of  the  cone  of  light  is  eliminated  and 
that  which  passes  through  the  slide  consists  of  rays  which  diverge 
but  slightly.  In  each  method  it  seems  to  be  expedient  to  use  as 
strong  an  illuminating  system  as  possible.  In  using  a  microscope 


OBSERVATIONS  IN  ORDINARY  LIGHT 


33 


without  an  Abbe  illuminating  apparatus  it  is  always  advisable 
to  leave  the  condenser  in  place  as  long  as  it  does  not  diminish  the 
field  of  vision,  although  this  is  contrary  to  the  usual  instructions. 
With  the  weakest  objectives,  however,  it  must,  in  all  cases,  be 
removed.  It  is  therefore  expedient  to  carry  out  all  investigations 
on  the  microscope  with  the  condenser  in  place  and  it  is  only 
removed  on  passing  over  to  the  lowest  magnifications. 

Differences  in  the  Indices  of  Refraction  Under  the  Microscope. — 
Since  complete  disappearance  of  a  crystal  in  a  liquid  is  only  to  be 
observed  when  the  indices  of  refraction  of  the  two  are  about 
equal,  and  since  a  crystal  stands  out  in  relief  from  the  surrounding 
medium  in  the  same  manner  whether  the  index  is  higher  or  lower, 
one  is  in  doubt  whether  to  ascribe  the  relief  to  one  or  the  other 


FIG.  44. — Higher 

Index  of  Crystal  in  Canada 


FIG.  45. — Lower 
with  Raised  Tube. 


cause.  The  determination  as  to  whether  the  index  of  the  crystal 
is  higher  or  lower  than  that  of  the  liquid,  however,  is  very  simple. 
The  cone  of  light  is  cut  down  until  the  contact  between  the 
crystal  and  the  liquid,  or  between  two  crystals,  appears  as  a 
sharp  line,  and  then  the  objective  is  raised.  A  distinct  band  of 
light  can  be  recognized  parallel  to  the  contact  between  the  two 
mediums  and  this  moves  toward  the  substance  with  the  higher  index 
upon  raising  the  tube,  while  the  more  weakly  refracting  object 
appears  to  have  a  dark  border.  Upon  lowering  the  tube,  the 
opposite  phenomenon  is  observed.1 

Fig.  44  shows  crystals  of  barium  nitrate  (n  =1.57)  in  Canada  balsam  with 
the  tube  raised.  The  band  of  light  can  be  distinctly  recognized  within  the 
edges  of  the  crystal.  The  opposite  is  the  case  in  Fig.  45,  which  shows  tetra- 

1  This  is  the  Becke  method. 
3 


34 


PETROGRAPHIC  METHODS 


hedrons  of  sodium  uranyl  acetate  having  a  lower  index  of  refraction  than 
the  surrounding  Canada  balsam,  and  therefore  when  the  tube  is  raised,  a 
sharp  bright  band  can  be  seen  outside,  and  a  dark  one  inside  the  crystal. 

This  phenomenon  can  be  explained  in  the  following  manner. 
All  the  rays  from  the  illuminating  apparatus  striking  obliquely 
on  the  contact  of  two  differently  refracting  substances,  Fig.  46, 
pass  from  the  medium  with  lower  index  on  the  left  into  the 
one  with  higher  index  on  the  right.  On 
the  other  hand,  only  a  portion  of  the 
light  will  pass  from  the  higher  refracting 
medium  into  the  lower,  because  within 
a  certain  angle  /?  total  reflection  of  the 
light  takes  place.  This  angle  is  de- 
pendent upon  the  ratio  of  the  indices  of 
refraction  in  the  two  bodies  and  is  larger 
the  greater  the  difference  between  the 
indices. 


FIG.  46.— Behavior  of  Rays  on  the  Con- 
tact between  two  Different  Media. 


FIG.  47. —  Observation 
through  the  Microscope. 


If  the  cone  of  light  is  narrowed  down  so  that  only  those  rays 
are  present  which  fall  within  the  angle  f)  it  is  evident  that  a 
strongly  illuminated  zone  will  appear  on  the  side  of  the  contact 
line  on  which  the  light  passing  through  the  border  is  added  to 
that  which  is  totally  reflected,  i.e.,  on  the  side  of  the  more 
strongly  refracting  substance,  and  on  the  other  side  where  no 
light  passes  through,  there  will  be  an  equally  pronounced  dark 
band.  Fig.  47  shows  diagrammatically  this  phenomenon  as 
observed  in  the  microscope.  If  the  ocular  is  placed  in  the 


OBSERVATIONS  IN  ORDINARY  LIGHT 


35 


plane  of  the  image  B,  the  contact  between  the  two  substances 
appears  as  a  sharp  line.  When  the  tube  is  raised  to  the  position 
B"  the  band  of  light  will  be  observed  on  the  side  of  the  substance 
with  the  higher  index  of  refraction,  due  to  the  inversion  of  the 
image,  and  when  the  tube  is  lowered  so  that  the  ocular  comes 
to  the  position  B',  the  light  band  appears  on  the  side  of  the 


FIG.  48.— High  FIG.  49. — Medium 

Index  of  Mineral  in  Canada  Balsam. 


FIG.  50. — Mineral  with  Low  Index  in  Canada  Balsam. 

lower  refracting  medium.  The  following  general  rule  may  be 
applied:  Upon  raising  the  tube  the  light  zone  moves  toward  the 
substance  with  the  higher  index  of  refraction. 

Figs.  48-50  show  this  phenomenon  as  ordinarily  observed  on 
a  thin  section  embedded  in  Canada  balsam.  If  a  mineral  like 
quartz  (n  =  1 . 54,  Fig.  49)  has  approximately  the  same  index 


36  PETROGRAPHIC  METHODS 

as  Canada  balsam,  the  unevenness  of  the  surface,  represented  by 
Fig.  38,  p.  22,  can  scarcely  be  observed  at  all,  if  very  nearly 
parallel  light  passes  through  the  slide.  Thin  sections  of  such 
colorless  minerals  are  scarcely  visible  when  the  illuminating 
apparatus  is  completely  lowered.  It  is  different  in  Figs.  48 
and  50  representing  slides  of  olivine,  ft  =  1.68,  and  haiiyne, 
ft  =  1.48,  respectively.  The  index  of  refraction  of  the  former  is 
considerably  higher,  that  of  the  latter  lower  than  Canada 
balsam,  ft  =  1.54.  The  irregularities  of  the  surface  are  filled 
up  with  a  medium  that  is  optically  different  and  therefore, 
when  the  illuminating  apparatus  is  lowered,  they  appear  very 
distinctly  because  various  points  on  the  surface  are  inclined 
at  different  angles  to  the  rays  of  light  which  are  deflected 
differently  and  some  are  totally  reflected.  The  result  is  the 
rough  chagrined  appearance  of  the  thin  section  under  investi- 
gation. Since  the  roughness  appears  more  readily  the  greater 
the  difference  of  index  of  refraction  between  the  mineral  and 
Canada  balsam,  the  amount  of  lowering  of  the  illuminating 
apparatus  or  of  closing  of  the  iris  diaphragm  necessary  to 
show  this  difference  is  an  excellent  means  for  the  approximate 
determination  of  the  index  of  refraction  of  minerals  in  thin 
sections.  Care  must  be  taken  that  the  slides  are  uniformly 
ground,  otherwise  there  will  be  great  differences  in  the  properties 
of  the  surfaces. 

If  isolated  crystals  are  studied  instead  of  thin  sections,  the  roughness  of 
the  surface  is  rarely  of  importance.  Observation  of  the  crystal  form  in 
ordinary  light  can  only  be  accomplished  when  there  is  a  great  difference 
between  the  indices  of  refraction  of  the  crystal  and  that  of  the  surrounding 
medium,  and  this  can  be  recognized  by  narrowing  up  the  cone  of  light 
from  below. 

If  a  grain  is  embedded  in  a  liquid  with  approximately  the  same  index  of 
refraction,  its  form  will  not  be  observed  at  all,  or  at  best  will  be  very  indis- 
tinct, even  when  the  cone  of  light  is  narrowed  down  as  much  as  possible. 
This  is  well  illustrated  by  quartz  embedded  in  Canada  balsam  (n  =  1 . 54) . 
If,  however,  the  quartz  crystal  is  embedded  in  linseed  oil  (n  =  1.48)  or 
monobrom-naphthalene  (n  =  1 . 66)  its  form  will  be  distinct  even  when  the 
illuminating  cone  is  much  wider. 

Determination  of  the  Index  of  Refraction. — Immersion  Method. 
—By  this  method  the  difference  in  the  indices  of  refraction  of  two 
mineral  grains  in  contact  with  one  another  in  a  thin  section  can 
be  determined  accurately  as  well  as  the  individual  indices  of 
refraction  of  isolated  crystals  or  fragments  embedded  in  a 


OBSERVATIONS  IN  ORDINARY  LIGHT  37 

liquid  of  known  index.     A  series  of  liquids   POTASSIUM  MERCURIC 

IODIDE  SOLUTIONS. 
called  indicators  is  arranged  so  that  the        gi  nj) 

index   of   each   exceeds    that   of  the  pre-  3.2  1.733 

ceding  one   by  0.01-0.02.      A    series    of  3.1  1.715 
potassium   mercuric  iodide  solutions  with 

different  concentrations  furnishes  a  scale,  2'g  i  658 

which  is  quite  easily  controlled,  and  the  2.7  1.640 

index    of    refraction    is    readily    obtained  2.6  1.621 

from  the  accompanying  table  by  determin-  2-5  1.602 
ing  the  specific  gravity. 

These  indicators  have  the  disadvantage  2'2  i  546 

that  the  index  of  refraction  changes  some-  2.1  1 . 527 

what   during   the    operation,    due    to    the  2.0  1.509 

evaporation  of  water.      Hence,  homogen-  1.491 

eous  liquids  of  definite  chemical  composi-  ™  1.473 

tion   are   better.      The  following  table  is  1'6 

given  by  Schroeder  van  der  Kolk:  1,5  1.419 

Water 1.33 

Ethyl  alcohol 1 .36 

Amyl  alcohol 1 . 40 

Chloroform 1 . 45 

Castor  oil 1 .48 

Benzol   1 . 50 

Monochlorbenzol 1 . 52 

Clove  oil.... 1.54 

Monobrombenzol 1 .56 

Aniline 1 . 58 

Bromoform 1 . 59 

Cinnamon  oil 1 . 60 

Carbon  disulphide 1 . 63 

a-Monochlor  naphthalene 1 . 64 

a-Monobrom  naphthalene 1 . 66 

Klein's  solution 1 . 70 

Thoulet  solution 1 . 73      . 

Methylene  iodide 1 . 75 

Solutions  with  intermediate  values  can  be  prepared  by  mixing 
these  indicators  and  this  process  is  continued  until  a  mixture 
has  been  found  in  which  the  crystal  disappears  as  completely  as 
possible.  Complete  disappearance  can  usually  not  be  obtained 
in  white  light,  on  account  of  the  difference  of  dispersion  be- 
tween the  crystal  and  liquid.  For  this  reason  it  is  better  to 
use  monochromatic  light.  When  the  proper  mixture  has  been 

XS  =  specific  gravity.nD  index  of  refraction  for  the  Na  line. 


38  PETROGRAPHIC  METHODS 

found,  its  index  of  refraction  should  be  determined  by  means  of 
a  Bertrand  total  reflectometer,  which  allows  a  very  accurate 
determination  of  the  index  to  be  made  rapidly,  using  only  a 
single  drop  of  the  liquid  for  this  purpose.  This  immersion 
method  gives  sufficiently  good  results  for  all  practical  purposes, 
provided  the  comparison  of  indices  of  the  liquid  and  mineral  is 
carried  out  according  to  the  methods  described,  pages  33-36. 

The  method  of  oblique  illumination  proposed  by  Schroeder 
van  der  Kolk  gives  more  accurate  results.  Oblique  illumination 
is  produced  either  by  placing  the  mirror  on  the  microscope 
obliquely  or  by  inserting  a  piece  of  cardboard  laterally  be- 
tween the  mirror  and  the  illuminating  apparatus.  In  the  larger 
microscopes  the  iris  diaphragm  can  be  moved  horizontally  or 
there  is  a  device  which  when  pushed  in  stops  the  light  eccentric- 


\ 

FIG.  51.  FIG.  52. 

Oblique  Illumination. 

Crystal  higher  Crystal  lower 

Index  than  the  Liquid. 

ally.  Usually,  however,  a  piece  of  cardboard  is  quite  efficient. 
Oblique  illumination  can  also  be  obtained  by  screening  one  side 
above  the  ocular.  An  Exner  refractometer  can  be  used  for  this 
purpose.  It  consists  of  a  hood  to  be  placed  over  the  ocular 
with  a  diaphragm  at  the  focal  distance  from  the  ocular.  This 
diaphragm  can  be  closed  laterally  by  means  of  a  slide.  If  a  grain 
is  immersed  in  a  liquid  it  affects  transmitted  parallel  light  in 
much  the  same  manner  as  a  converging  lens,  i.e.,  it  converges 
the  rays  if  its  index  is  higher  than  that  of  the  liquid  and  in  the 
opposite  case  it  causes  them  to  be  divergent.  If  a  bundle  of 
parallel  oblique  rays  passes  through  the  preparation,  only  that 
light  is  taken  up  by  the  objective,  in  the  first  case,  which  is  on 
the  same  side  as  the  screen,  Fig.  51,  while  in  the  second  case,  only 
that  on  the  opposite  side  is  received  by  the  objective,  Fig.  52. 


OBSERVATIONS  IN  ORDINARY  LIGHT  39 

However,  since  the  objective  produces  inversion,  the  reverse 
phenomenon  is  seen  in  the  microscope.  //  the  grain  has  a 
higher  index  of  refraction  than  the  liquid,  the  dark  band  is  seen 
on  the  side  next  to  the  screen,  but  if  the  grain  has  a  lower  index 
it  appears  on  the  opposite  side. 

This  method  is  extremely  delicate  when  liquid  and  crystal 
have  very  nearly  the  same  indices,  and  especially  when  their 
dispersions  are  at  the  same  time  quite  different.  It  may  happen 
that  the  mineral,  which  generally  has  the  weaker  dispersion,  will 
possess  a  lower  index  for  blue  and  a  higher  index  for  red  than  the 
liquid.  A  brilliant  colored  border,  blue  on  the  side  of  the  screen 
and  red  on  the  opposite  side,  is  an  indication  that  liquid  and  min- 
eral have  very  nearly  the  same  indices  of  refraction.  In  rare 
cases,  when  the  mineral  has  a  stronger  dispersion  than  the  liquid, 
the  position  of  the  colors  is  reversed.  To  determine  the  indices 
of  refraction  of  double  refracting  crystals  by  this  method,  it' 
must  be  possible  to  make  a  determination  of  each  index  succes- 
sively. The  ordinary  ray  of  uniaxial  crystals  can  be  recognized 
in  polarized  light,  after  its  vibration  direction  has  been  placed 
parallel  to  that  of  the  polarizer,  in  that  the  optical  reaction  is 
very  sharp  even  though  the  grains  be  very  irregular,  because  the 
refraction  of  the  ordinary  ray  is  the  same  in  all  directions.  The 
extraordinary  ray,  however,  cannot  be  determined  as  accurately 
especially  in  substances  which  have  no  good  cleavage  because  it 
appears  at  every  point  on  the  surface  with  different  values  due 
to  the  different  inclinations  of  the  surface  toward  the  optic  axis. 
To  determine  the  principal  indices  of  refraction  of  double  refracting 
substances  accurately,  a  section  must  be  taken  whose  orientation 
has  been  previously  determined  in  convergent  polarized  light. 

A  center  screen  is  very  serviceable  for  observing  the  differences  between 
the  indices.  It  causes  the  object  to  be  illuminated  by  the  strongly  diverging 
outer  rays  from  the  illuminating  apparatus  and  can  be  placed  in  a  slide 
in  the  polarizer,  but  must  be  adapted  for  the  angle  of  aperture  of  the  ocular. 
This  method  is  especially  useful  in  the  investigation  of  minute  objects, 
and  in  the  study  of  inclusions  and  so  forth. 

An  accurate  method  for  determining  the  index  of  refraction  of  micro- 
scopic objects,  consisting  of  a  combination  of  an  Abbe  total  reflectometer 
with  a  microscope,  was  suggested  by  C.  Klein.  By  this  method  all  indices 
of  refraction  in  any  microscopic  cross  section  of  a  mineral  can  be  determined, 
but  the  part  of  the  slide  surrounding  this  section  must  be  covered  with 
black  varnish,  and  good  results  can  be  obtained  even  with  comparatively 
small  individuals.  The  method  of  observation  is  the  same  as  with  an 
Abbe  total  reflectometer  itself  and  need  not  be  discussed  further  here.  It 


40  PETROGRAPHIC  METHODS 

need  only  be  stated  that  the  stand  upon  which  the  hemisphere  of  the  Abbe 
apparatus  rests  is  hollow,  the  hemisphere  itself  is  placed  over  the  opening 
on  a  ground  plane  surface,  so  that  the  object  to  be  investigated  can  be 
observed  on  the  microscope  in  transmitted  ordinary  or  polarized  light. 
However,  this  apparatus,  which  is  quite  complicated  and  comparatively 
expensive,  has  not  been  used  as  extensively  as  it  deserves  to  be. 

The  total  reflectometer  constructed  by  Wallerant  is  somewhat  simpler 
and  can  be  attached  to  any  microscope.  It  consists  of  a  plate  to  be  set 
on  the  stage  and  carries  a  prism  of  strongly  refracting  glass,  which  can  be 
rotated  about  a  horizontal  axis  parallel  to  its  refracting  edge,  either  alone 
or  in  combination  with  an  alidade  reading  to  2'.  The  edge  itself  is  ground 
off  parallel  to  the  base  of  the  prism,  so  that  when  the  apparatus  is  used 
on  the  microscope,  light  is  transmitted  through  it.  An  uncovered  thin 
section,  polished  as  perfectly  as  possible,  is  firmly  pressed  on  the  base  of 
the  prism  by  two  clamps,  the  contact  being  made  with  methylene  iodide. 
The  extinction  directions  of  the  section  are  then  determined  in  transmitted 
light.  The  prism  is  now  rotated  until  convergent  rays  from  a  powerful 
source  of  light  are  incident  upon  one  of  the  lateral  faces.  The  light  is 
reflected  by  the  section  and  emerges  from  the  other  surface  of  the  prism  in 
the  axis  of  the  microscope.  The  object  is  centered  as  accurately  as  possible 
and  the  vibration  directions  as  previously  determined  by  the  extinction 
are  brought  into  position,  and  the  prism  rotated  until  the  distribution  of 
light  and  shadow  indicate  the  critical  angle  for  total  reflection.  The  iris 
diaphragm  in  the  tube  is  now  closed  until  only  the  section  to  be  studied 
is  visible.  Now  the  ocular  is  replaced  by  a  telescope  containing  a  spec- 
troscope, which  isolates  the  yellow  rays  from  the  white  light.  The  edge  of 
the  section  is  then  placed  parallel  to  a  cross  hair  in  the  telescope  and  a 
reading  taken.  Then  the  cross  hairs  are  illuminated  by  light  sent  in  from 
the  side  by  means  of  a  total  reflecting  prism.  The  prism  bearing  the 
section  is  rotated  with  the  alidade  until  the  image  of  the  cross  hairs  reflected 
from  its  base  coincides  with  the  object  A  second  reading  is  then  made 
and  the  angle  of  total  reflection  determined. 

The  liquids  employed  so  frequently  in  organic  microscopy  to  increase  the 
illumination  of  an  object  depend  upon  the  principle  of  compensation  of  the 
refraction  of  light.  Preparations  of  crystals  and  rocks  are  generally 
embedded  in  Canada  balsam,  the  index  of  which  may  vary  between  1 . 53 
and  1 . 545,  depending  upon  the  amount  of  evaporation.  Canada  balsam 
is  especially  adapted  for  this  purpose  because  it  is  quite  liquid  when  fresh 
and  upon  heating  slightly  is  transformed  into  a  clear  transparent  almost 
co  orless  cement  that  sticks  firmly  and  becomes  very  slightly  double 
refracting  if  at  all,  upon  cooling  as  other  resins  generally  do.  Solutions  of 
hardened  Canada  balsam  in  benzol,  xylol,  etc.,  are  sometimes  used  especially 
with  substances  that  cannot  be  warmed.  The  index  of  refraction  is  then 
somewhat  less  on  account  of  the  dilution.  For  chemists  Canada  balsam  has 
a  disadvantage  in  that  it  is  a  very  good  solvent  not  only  for  organic  sub- 
stances, but  also  for  numerous  inorganic  salts,  and  its  solvent  action  is 
sometimes  increased  by  the  addition  of  benzol.  A  more  feebly  refracting 
cement  is  needed  in  rare  instances  and  then  linseed  oil  is  employed,  n  =  l .  48. 
When  placed  in  the  direct  rays  of  the  sun  for  a  few  weeks  it  bleaches  and 
polymerizes. 


OBSERVATIONS  IN  ORDINARY  LIGHT 


41 


Determination  of  Form  and  Cleavage. — A  stereoscopic  micro- 
scope, with  which  an  object  can  be  observed  as  a  body,  is  well 
adapted  for  determining  the  form-  of  crystals  and  especially  for 
the  investigation  of  etch  figures  that  are  not  very  deep.  The 
distinctness  of  form  that  can  be  obtained,  especially  with  a  Zeiss 
binocular  microscope  simplifies  greatly  the  deciphering  of  micro- 
scopic crystals.  The  model  shown  in  Fig.  53,  constructed  by  the 


FIG.  53. — Grenough  Binocular  Microscope  by  C.  Zeiss  in  Jena. 

above  firm  according  to  the  principle  of  Grenough,  has  the  advan- 
tage that  the  image  of  the  crystal  appears  in  its  true  position 
due  to  prisms  in  the  tubes.  Since  there  are  two  objectives,  each 
in  the  axis  of  an  observation  tube,  strong  magnification  cannot 
be  used.  Other  devices  for  stereoscopic  investigation  with  a 
microscope,  such  as  a  double  ocular  or  stereoscopic  ocular  which 
can  be  placed  on  a  simple  microscope,  give  good  results  only  with 
low  magnification,  but  they  do  not  at  the  same  time  produce  the 
distinctness  of  form  obtained  by  a  binocular  microscope.  The 


42  PETROGRAPHIC  METHODS 

importance  of  stereoscopic  observation  is  confined  to  a  small 
number  of  cases  and  the  binocular  microscope  cannot  replace  a 
polarizing  microscope.  The  next  step  in  the  microscopic 
determination  of  crystals  is  to  observe  and  sketch  the  form  as 
accurately  as  possible.  (See  methods  for  sketching  crystals  at 
the  end  of  Part  I.)  This  must  not  be  confined  to  the  determi- 
nation of  the  outline  alone;  but  care  must  be  taken  to  decipher 
as  well  as  possible  the  properties  of  the  crystal  as  a  whole. 
Without  this,  entirely  erroneous  conclusions  may  be  reached 
because  the  angles  between  the  edges  often  appear  greatly  dis- 
torted in  consequence  of  the  irregular  position  of  a  crystal  in  the 
slide. 

The  measurement  of  characteristic  angles  goes  hand  in  hand 
with  the  determination  of  crystal  form,  but  only  such  angles  are 
to  be  considered  as  are  bounded  by  edges  lying  exactly  in  the 
plane  of  the  stage,  or  by  planes  perpendicular  to  this  direction. 

One  leg  of  the  angle  to  be  measured  is  placed  parallel  to  one  of  the  cross 
hairs  and  a  reading  of  the  position  of  the  stage  taken.  Then  the  stage  is 
rotated  until  the  other  leg  is  parallel  -to  the  same  cross  hair  and  another 
reading  taken,  the  difference  between  the  two  being  the  angle  sought. 
The  edge  can  be  placed  in  parallel  position  much  more  perfectly,  the  longer 
and  straighter  it  is  and  the  more  aplanatic  the  ocular  lenses  are.  It  seems 
better  too  in  making  such  measurements,  not  to  make  the  edge  coincide 


FIG.  54a.  FIG.  54b.  FIG.  54c. 

Measurement  of  Angles  with  a  Lesson's  Prism. 

exactly  with  the  cross  hair,  but  to  leave  a  small  space  between  the  two  so 
that  the  parallel  position  can  be  found  much  more  accurately.  In  favor- 
able cases,  if  the  position  is  not  perfect,  the  reading  to  whole  degrees  and  the 
estimating  to  quarter  degrees  are  quite  sufficient. 

A  Leeson's  prism  can  be  advantageously  used  for  the  accurate  measure- 
ment of  plane  angles,  especially  on  very  small  crystals.  It  consists  of  an 
achromatic  quartz  prism  that  is  placed  over  the  ocular  so  that  it  can  be 
rotated  in  a  graduated  holder.  With  it,  two  images  of  the  object  partially 
overlying  each  other  are  seen,  Fig.  54a.  The  prism  is  rotated  until  one 
edge  of  the  angle  to  be  measured  coincides  in  both  images,  Fig.  546.  A 
reading  of  the  scale  is  then  made  and  the  other  edge  treated  in  the  same 
manner,  Fig.  54c.  More  accurate  results  can  also  be  obtained  when  a 
glass  plate  with  a  system  of  fine  parallel  lines  etched  upon  it  is  set  at  the 
focus  of  the  ocular  in  place  of  the  cross  hairs.  A  long  line  crosses  the 


OBSERVATIONS  IN  ORDINARY  LIGHT 


43 


middle  of  the  system  perpendicularly.  This  and  the  middle  line  of  the 
system  are  somewhat  heavier  so  that  the  object  can  be  easily  centered. 
The  edges  of  the  angle  to  be  measured  are  then  brought  into  parallel 
position  with  the  fine  lines,  one  after  the  other. 

An  ocular  goniometer  also  deserves  some  mention  at  this  time.  This  is 
an  ocular  in  which  one  of  the  cross  hairs  can  be  rotated  with  respect  to  the 
other,  and  the  amount  of  rotation  read  on  a  circular  scale.  The  movable 
hair  is  placed  parallel  successively  to  the  legs  of  the  angle  to  be  measured. 
This  method  has  the  advantage  that  the  slide  need  not  be  moved.  Micro- 
scopes in  which  the  nicols  may  be  rotated  simultaneously  possess  the 
advantages  of  an  ocular  goniometer  inasmuch  as  the  cross  hairs  also  rotate 
with  respect  to  the  object. 

Actual  reflection  goniometric  measurements  can  be  made  with  a  micro- 
scope if  a  small  reflection  goniometer  or  the  rotating  apparatus,  to  be 
described  in  the  appendix,  is  combined  with  a  microscope.  However, 
such  microscopic-goniometric  measurements  will  only  give  good  results 
after  considerable  practice  and  after  all,  are  of  very  little  use.  If  in  such 
investigations  the  microscope  itself  is  to  be  used  to  observe  the  reflection 
of  the  surfaces,  a  Gauss  mirror  appliance  is  valuable.  It  consists  of  a 


FIG.  55.— Distorted  Crystal. 


FIG.  56. — Normal  Development. 


mirror  placed  obliquely  on  the  ocular  and  a  black  cross  hair  under  the 
objective.  After  the  surface  has  been  placed  approximately  perpendicular 
to  the  axis  of  the  microscope,  the  tube  is  lowered  half  the  focal  distance  of 
the  objective  and  the  mirror  image  of  the  cross  hairs  is  seen  in  the  ocular. 
When  the  surface  is  accurately  adjusted  the  image  coincides  with  the 
cross  hairs.  This  device  can  only  be  used  with  the  lowest  power  objectives. 

The  form  of  crystals  observed  in  the  microscope  is  frequently  distorted 
in  many  ways,  especially  in  cases  of  rapid  crystallization,  so  that  the 
image  of  a  crystal  can  be  deciphered  only  with  difficulty  on  account  of  the 
irregular  development  of  similar  faces. ' 

In  such  cases,  however,  the  angles  between  the  edges  are  always  constant 
and  a  measurement  of  them  affords  a  means  of  finding  the  faces  belonging 
to  a  form  and  of  recognizing  the  symmetry  of  the  crystal.  Fig.  55  represents 
a  cross  section  of  a  distorted  crystal,  which  at  first  glance  appears  entirely 
unsymmetrical.  Measurement  of  the  plane  angles  shows  that  the  angles 
be  and  b'c'  are  both  approximately  90°,  while  the  other  angles  have  equiva- 
lents among  themselves.  It  is  presumably  an  orthorhombic  crystal  in 
which  the  prism  faces  be  and  b'c'  have  been  developed  unequally,  Fig.  56. 


44 


PETROGRAPHIC  METHODS 


Rapid  crystallization  produces  in  many  cases  imperfect  growth  and  de- 
pressions bounded  by  more  or  less  regular  forms  appear  on  the  surface  of 
crystals.  The  cause  is  more  rapid  growth  on  the  edges  than  in  the  middle  of 
the  faces.  These  irregularities  pass  over  into  skeletal  crystals,  Fig.  57, 
which  may  be  developed  in  a  large  variety  of  delicate  forms.  Star-shaped 


FIG.  57. — Skeletal  Crystals  of  Olivine. 

skeletal  crystals  of  snow,  ice  flowers,  etc.,  are  the  best  known  forms  of  this 
type.  These  skeletal  forms  in  turn  pass  over  into  minute  growths  resem- 
bling arrow  heads,  rods,  etc.,  which  cannot  be  definitely  determined  as 
crystals  and  are  called  crystallites,  Fig.  58. 


f  „  Mi,  i 
*  <$$  i 


FIG.  58.— Crystallites. 
a,  Globulete;  b,  Margarite;  c,  Cumulite;  d,  Baculite;   e,  and/,  Trichite. 

Minerals  observed  in  thin  sections  very  frequently  show  more 
or  less  regular  cracks  which  indicate  cleavage.  In  many  cases 
a  few  straight  sharp  cracks  cut  across  the  whole  section,  for  ex- 
ample, mica,  Fig.  59.  The  cleavage  is  perfect  even  though  the 
number  of  cracks  is  small.  The  cracks  may  be  discontinuous, 


FIG.  59-61.— Perfect. 


Distinct. 


Imperfect. 


Cleavage. 

as  in  hornblende,  Fig.  60.  Here  the  cleavage  is  distinct,  or  good. 
If  the  cracks  are  more  or  less  curved,  but  following  on  the  whole 
certain  directions,  the  cleavage  is  said  to  be  imperfect,  garnet, 
Fig.  61.  Different  systems  of  cleavage,  of  equal  or  unequal 
value,  may  be  present  enclosing  a  characteristic  angle,  which  can 


OBSERVATIONS  IN  ORDINARY  LIGHT  45 

be  determined  by  measurement.  Much  more  importance  is 
generally  ascribed  to  the  measurement  of  cleavage  angles  in  thin 
sections  than  it  really  deserves,  because  the  size  of  the  angles 
is  largely  dependent  upon  the  orientation  of  the  various  cross 
sections  of  a  crystal  and  this  of  course  is  variable.  In  any  case, 
it  must  be  shown  by  raising  and  lowering  the  tube  that  the  cleav- 
age cracks  traverse  the  section  as  nearly  vertically  as  possible. 

Investigation  of  cleavage  fragments  of  crystals  or  of  rock- 
forming  minerals  often  gives  very  characteristic  results,  and  it  is 
often  serviceable  for  rapid  orientation  or  for  supplementing  the 
observations  on  thin  sections.  . 

In  isolated  crystals,  cleavage  cracks  are  not  often  observed 
and  then  only  if  the  cleavage  is  very  perfect.  Sometimes 
crystals  can  be  crushed  and  cleavage  fragments  formed,  the  form 
of  which  is  a  more  characteristic  feature  of  the  substance  than 
can  be  found  in  a  thin  section.  Cracks  parallel  to  gliding  planes 
may  appear  similar  to  cleavage  cracks  in  microscopic  objects 
and  there  is  no  way  of  distinguishing  them.  Occasionally  more 
or  less  regular  parting,  which  may  be  due  to  various  causes, 
appears  quite  similar  to  cleavage  cracks. 

Measurement  of  Size  and  Thickness. — While  it  is  frequently 
interesting  to  know  the  size  of  an  object,  its  thickness  is  more 
often  determined.  A  micrometer,  which  is  sometimes  placed  on 
the  stage  and  sometimes  in  the  ocular,  is  used  for  the  determina- 
tion of  size.  When  placed  on  the  stage,  it  gives  the  size  directly, 
but  when  in  the  ocular  the  results  must  be  recalculated  for  the 
objective  used  and  for  the  length  of  the  tube.  On  the  other  hand, 
errors  made  in  the  object  micrometer  divisions  are  increased  by 
the  total  magnification  of  the  instrument,  while  those  of  the 
ocular  micrometer  are  only  increased  by  the  magnification  of  the 
ocular  itself. 

In  certain  cases  a  scale  on  the  mechanical  stage,  Fig.  34, 
page  19,  in  which  divisions  on  the  drum  of  the  screw  permit 
1/1000  mm.  to  be  estimated,  serves  as  an  object  micrometer. 
The  small  errors,  which  are  apt  to  occur  in  cutting  the  thread 
of  the  screw,  make  such  operations  unreliable. 

Divisions  of  only  1/10  mm.  are  demanded  of  a  micrometer 
placed  in  the  ocular  so  that  the  requirements  of  the  execution  are 
much  less  and  errors  likewise  smaller.  Such  an  ocular  microm- 
eter can  simply  be  placed  in  the  ocular  at  the  focal  distance, 
or  a  micrometer  ocular  can  be  constructed  in  such  a  manner  that 


46  PETROGRAPHIC  METHODS 

a  cross  hair  at  the  focus  of  the  ocular  can  be  moved  by  means  of  a 
screw  with  a  measuring  drum  working  at  right  angles  to  the  axis 
of  the  instrument,  while  an  immovable  cross  hair  indicates  the 
center  of  the  field.  In  other  cases  the  scale  itself  is  fixed  and  a 
Ramsden  ocular  with  a  central  division  is  moved  over  the  scale 
by  means  of  a  measuring  drum.  The  division  to  which  this 
mark  must  be  brought  as  nearly  as  possible,  is  made  upon  the 
upper  plane  surface  of  a  lens,  which  is  in  turn  adjusted  for  the 
objective  so  that  the  eccentricity  produced  by  lateral  displace- 
ment of  the  ocular  is  compensated. 

A  micrometer  possessing  divisions  as  represented  in  Fig.  4, 
page  2,  and  placed  in  the  ocular,  may  serve  to  determine  the  rela 
tive  amounts  of  minerals  in  thin  sections.  Such  an  adjustment 
is  called  a  planimeter  ocular. 

The  measurement  of  the  thickness  of  microscopic  objects  is 
generally  possible  only  when  the  optical  properties  are  quite 
accurately  known.  Good  results  can  seldom  be  obtained  with 
isolated  crystals,  but  frequently  in  thin  sections  quite  accurate 
determinations  of  the  thickness  are  possible  because  certain 

known  minerals  are  nearly  always 
present  and  their  thickness  can  be 
estimated  by  means  of  the  inter- 
ference colors.  The  measurement  of 
the  thickness  is  not  very  reliable  with 
other  methods  even  in  the  most 


s\/r  favorable  cases. 

io 

A  method,   which  was    frequently 

used  formerly,   is   that   of   Duke   de 

FIG.  62,-D^deChauines        Chaulnes    for    the    determination    of 

the  index  of  refraction.      It  can  be 

also  used  in  reversed  manner  for  measuring  the  thickness. 
A  point  o,  Fig.  62,  is  sharply  focused  and  a  plane  parallel  plate 
of  a  transparent  substance  is  inserted  between  it  and  the  ob- 
jective. In  consequence  of  the  refraction  of  the  rays  or  and  os 
one  receives  the  impression  that  the  light  emanates  from  o' ' .  The 
objective  must  then  be  raised  by  means  of  the  micrometer  screw 
until  it  is  focused  on  o'.  With  a  known  thickness  of  the  plate,  the 
distance  oof  is  dependent  upon  its  index  of  refraction.  If  the 
latter  is  known,  we  have  a  basis  for  estimating  the  thickness  of 
the  plate.  The  change  of  focus  of  the  objective  can  be  read 
directly  from  the  divisions  on  the  drum  of  the  micrometer  screw. 


OBSERVATIONS  IN  ORDINARY  LIGHT  47 

This  method  is  often  used  but  care  must  be  exercised  not  to  overestimate 
the  accuracy  of  it  because  a  great  deal  of  practice  is  necessary  to  obtain 
reliable  results.  A  small  foreign  body,  e.g.,  a  dust  particle,  which  occurs 
between  the  object  and  the  cover-glass  is  focused  and  then  the  apparatus 
lowered  until  the  same  signal  is  seen  reflected  from  the  under  side  of  the 
slide.  The  amount  of  lowering  of  the  objective  constitutes  the  value  6  and 
the  thickness  of  the  slide  is  given  by  the  equation  d  =  bn,  where  n  is  the 
index  of  refraction  of  the  section  measured. 

The  measurements  are  most  accurate  when  stronger  magnifications  are 
used  because  the  amount  of  displacement  of  the  real  image  from  the  ob- 
jective increases  with  an  increase  of  magnification.  The  image  of  the 
point  chosen  must  be  brought  exactly  into  the  plane  of  the  cross  hairs  and 
the  proper  adjustment  can  be  recognized  when,  upon  moving  the  eye  above 
the  ocular,  the  image  and  cross  hairs  do  not  show  relative  displacement. 
Since,  in  general,  the  index  of  refraction  n  is  not  positively  known  and  con- 
stitutes the  greatest  error  in  this  method,  an  immersion  system  can  be 
employed  with  good  results  for  this  determination.  Then  it  is  not  neces- 
sary to  fix  the  value  of  the  index  of  refraction  itself,  but  only  to  determine 
the  difference  between  the  index  of  the  mineral  and  the  immersion  oil. 

Methods  that  depend  upon  observations  of  the  interference  colors  are 
much  more  exact.  There  may  be  well  orientated  sections  of  minerals 
present  in  the  slide  with  known  indices  of  refraction  from  which  their 
thickness,  which  indicates  the  thickness  of  the  slide,  can  be  estimated. 
Such  cross  sections  can  be  introduced  into  the  slide  artifically  by  cementing 
cleavage  plates  of  barite  on  different  sides  of  the  rock  section  and  grinding 
them  down  with  the  section.  Measurements  thus  made  are  at  any  rate  a 
close  approximation. 

Inclusions.— The  presence  of  inclusions  is  of  importance  in 
certain  cases  because  a  large  number  of  crystallized  bodies,  es- 
pecialy  minerals,  possess  the  ability  to  enclose  large  amounts 
of  foreign  substances  upon  crystallization.  This  is  often  so 
extensive  that  the  greater  part  of  the  crystal  consists  of  such 
foreign  bodies.  These  inclusions  can  be  recognized  in  ordinary 
light  by  the  differences  in  the  indices  of  refraction  and  color. 

Some  of  the  inclusions  occurring  in  this  way  are  crystallized'  substances 
and  must  be  determined  according  to  the  ordinary  methods.  Inclusions  of 
gas,  liquid,  and  glass,  which  frequently  furnish  important  clues  to  the 
genetic  relations,  are  also  found.  Gas  inclusions  in  crystallized  bodies 
always  stand  out  in  strong  relief,  because  the  index  of  refraction  of  the  gas 
is  far  below  that  of  the  crystallized  substance.  In  parallel  light  it  is  sur- 
rounded by  a  single  broad,  dark  band  produced  by  refraction  and  total 
reflection,  Fig.  63.  Separation  into  different  zones  is  never  observed  in 
gas  bubbles  because  all  gases  are  miscible.  In  observing  gas  inclusions 
care  must  be  exercised  to  note  that  the  gas  bubble  is  really  in  the  object 
and  not  in  the  Canada  balsam.  Air  bubbles  in  the  balsam  naturally  appear 
very  similar  to  gas  inclusions,  but  by  sharp  focusing  with  the  micrometer 
screw  they  can  be  seen  to  lie  either  above  or  under  the  object  itself. 


48 


PETROGRAPHIC  METHODS 


Liquid  and  glass  inclusions  on  the  other  hand  appear  to  be  separated  into 
different  zones.  The  liquids  include  salt  solutions,  water,  and,  in  some 
minerals,  liquid  carbon  dioxide.  In  all  cases  a  small  bubble  of  gas  or  air 
is  present  in  the  liquid,  which  under  certain  conditions,  especially  in  very 
small  inclusions  and  under  variations  of  temperature,  moves  about  quite 
perceptibly.  The  index  of  refraction  of  all  liquids  existing  in  nature  lies 
between  that  of  the  mineral  and  the  gas  bubble,  so  that  the  edges  of  both 
liquid  and  bubble  appear  as  heavy  lines  in  parallel  light,  Fig.  64.  Some- 


FIG.  63. 
Gas  Inclusions. 


FIG.  64. 
Liquid  Inclusions. 


FIG.  65. 
Glass  Inclusions. 


times  liquid  inclusions  seem  to  consist  of  several  immiscible  liquids  with 
different  indices  of  refraction  beside  each  other,  or  they  may  contain 
small  crystals.  If  the  liquid  in  such  an  inclusion  is  carbon  dioxide  it  can 
be  recognized  by  heating  the  preparation  for  some  time  a  trifle  above  the 
critical  temperature  of  carbon  dioxide.  The  bubble  will  disappear  and  the 
border  of  the  inclusion  will  become  darker. 

Glass  inclusions  likewise  contain  one  or  more  bubbles,  but  they  are  always 
immovable  and  do  not  show  such  heavy  contours  compared  with  the  in- 
cluding crystal  if  the  latter  has  a  low  index  of  refraction.  They  appear 
more  distinct  the  higher  the  index  of  refraction  of  the  crystal.  The  bubble 
itself  is  always  surrounded  by  a  broad  dark 
band,  Fig.  65.  A  regular  arrangement  of  inclu- 
sions is  very  characteristic  in  many  cases,  for 
example,  Fig.  66,  leucite. 


FIG.  66. — Leucite  with 
Slag  Inclusions. 


Color. — Another  observation  that  can  be 
made  in  ordinary  light  is  that  of  the  color. 
Particulars  concerning  ih&  naming  of  the 
colors  are  scarcely  necessary.  It  may  be 
pointed  out  that  many  bodies,  which  are 
very  dark  or  entirely  opaque  in  large 
crystals  are  colorless  or  nearly  so  in  thin  section,  for  example, 
augite,  while  others  which  are  very  light  colored  in  macroscopic 
crystals  show  their  color  quite  distinctly  in  the  thinnest  sections, 
thus,  andalusite  and  cyanite.  It  need  not  be  further  emphasized 
that  a  layer  of  any  substance  appears  lighter  colored  the  thinner 
it  is.  The  thinnest  sections  are  generally  not  chosen  for  deter- 
mining the  color. 

When  light  passes  through  a  body  it  suffers  diminution  of 
intensity  and  is  either  partially  or  entirely  absorbed.     If  the 


OBSERVATIONS  IN  ORDINARY  LIGHT  49 

absorption  is  only  slight  and  is  about  the  same  for  the  different 
colors  of  the  spectrum,  the  body  appears  colorless  and  trans- 
parent, but  if  the  absorption  is  complete  even  in  the  thinnest 
sections,  the  body  is  said  to  be  opaque.  If  certain  colors  of  the 
spectrum  are  absorbed  more  than  others  the  substance  appears 
colored.  Distinction  is  made  between  characteristic  colors,  or 
those  belonging  to  the  substance  itself,  and  dilute  colors  produced 
by  intermixture  of  foreign  material  present  in  such  a  fine  state 
of  division  that  it  cannot  be  detected  with  the  strongest  magni- 
fication. Coloring  produced  by  inclusions,  on  the  other  hand, 
can  always  be  detected  in  this  manner. 

Since  the  naming  of  the  colors  is  in  general  very  subjective,  attempts 
have  been  made  to  express  them  accurately  by  means  of  comparison  with 
an  objective  color  scale.  A  Radde  color  scale  is  especially  serviceable  for 
this  purpose.  It  shows  the  various  colors  in  a  horizontal  series  while  the 
various  shades  of  each  are  arranged  in  the  vertical  columns.  Spectroscopic 
dispersion  of  the  colors  obtained  with  a  spectroscope-ocular  is  of  very  little 
value.  See  page  49  concerning  false  colors,  pseudochroism,  which  is  often 
observed  on  small  opaque  bodies  with  strong  objectives.  The  color  of 
thin  plates,  which  are  in  themselves  colorless,  belongs  in  the  same  category. 
It  is  most  beautifully  observed  in  preparations  of  diatoms. 

Observations  in  Reflected  Light. — These  observations  are  con- 
fined, in  general,  to  surface  properties,  which  cannot  be  deter- 
mined in  transmitted  light,  such  as  the  luster  of  opaque  sub- 
stances, metallic  luster,  etch  figures,  etc.  Light  coming  in  from 
below  is  cut  out  for  this  purpose  and  only  light  falling  on  the 
object  from  above  is  used,  but  it  can  be  increased  by  reflectors 
or  converging  lenses. 

If  light  incident  on  the  preparation  from  above  is  used,  it  is  desirable  to 
use  an  objective  with  long  focal  length  because  otherwise  the  objective  holder 
cuts  out  a  large  portion  of  the  light.     Low  power 
objectives  are  therefore    more  serviceable  for  such 
observations  and  if  stronger  magnification  is  needed, 
in  spite  of  the  loss  of  light,  stronger  oculars,  which 
are  supplied   almost  entirely  for  this  purpose  with 
'the  larger  instruments,  are  used. 

If  objectives  of  shorter  focal  length  are  employed, 
a  vertical  illuminator  inserted  above  the  objective  is 
used.  Fig.  67  gives  a  cross  section  of  such  an  illumi-  JTIG  57 

nator.     It  consists  of  a  total  reflecting  prism  K  wliich      Vertical  Illuminator, 
sends  the  light  L,  coming  in  through  the  opening  D, 

down  through  the  objective  O  upon  the  preparation.     The  prism  covers 
one  half  of  the  objective  and  leaves  the  other  half  free  for  observation 
4 


50  PETROGRAPHIC  METHODS 

and  through  it  the  light,  reflected  from  the  object,  reaches  the  eye.  It 
must  be  noted  that,  in  using  this  method,  the  object  must  not  be  covered 
with  a  cover-glass  because  it  would  reflect  the  greatest  part  of  the  light. 
Uncovered  preparations  are  best  for  investigations  in  reflected  light.  A 
vertical  illuminator  cannot  be  used  with  the  stronger  objectives  on  ac- 
count of  the  great  loss  of  light  produced  by  the  double  transit  of  the  light 
through  the  objective. 


CHAPTER  IV 
Observations  in  Parallel  Polarized  Light 

Observations  in  polarized  light  are  of  especial  importance  in 
the  investigation  of  crystallized  bodies  because  by  this  means 
the  inner  structure  of  a  crystal  can  be  determined.  For  this 
purpose  a  nicol  prism,  the  polarizer,  is  placed  between  the  source 
of  light  and  the  object  to  be  studied.  By  rotating  the  stage 
through  360°  the  phenomena  can  be  observed  which  a  crystal 
shows  in  various  planes  in  which  the  plane  polarized  light 
from  the  polarizer  is  vibrating.  A  second  nicol  prism,  the 
analyzer,  with  its  vibration  direction  exactly  at  right  angles  to 
that  of  the  polarizer  may  be  placed  above  the  objective  and 
the  phenomena  of  interference  of  light  between  crossed  nicols 
may  then  be  observed.  Before  the  phenomena  thus  observed 
can  be  discussed  in  detail,  the  optical  properties  of  crystals 
must  be  reviewed  briefly. 

i.  Optical  Properties  of  Crystals 

Single  and  Double  Refraction. — If  a  small  illuminated  open- 
ing in  a  dark  screen  is  viewed  perpendicularly  through  a  cleavage 
piece  or  a  plane  parallel  plate  cut  in  any  direction  from  a  crystal 
of  fluorite  or  halite,  it  appears  in  the  same  place  and  in  the 
same  manner  as  it  would  if  the  plate  were  not  there.  If  instead 
of  the  plane  parallel  plate  a  prism  of  the  -same  substance  is 
used  and  the  opening  is  viewed  through  two  surfaces  oblique  to 
each  other,  the  image  will  appear  removed  from  its  original 
position  and  will  be  surrounded  by  a  colored  border,  but  only  one 
image  of  the  opening  will  be  seen.  All  amorphous  bodies  and 
crystals  of  the  cubic  system  behave  in  the  same  manner.  They 
are  called  singly  refracting  or  isotropic  substances. 

If  the  index  of  refraction  is  determined  by  the  deviation  of 
light  in  a  series  of  such  prisms  of  a  singly  refracting  substance, 
it  will  be  found  to  be  the  same  regardless  of  the  orientation 
of  the  prism  in  the  crystal.  Singly  refracting  bodies  deviate 

51 


52 


PETROGRAPHIC  METHODS 


light  in  all  directions  in  the  same  manner.  They  are  there- 
fore called  isotropic.  Since  the  refraction  of  light  in  a  prism 
is  dependent  upon  the  index  of  refraction  of  the  substance  in 
question  and,  since  the  index  is  inversely  proportional  to  the 
velocity  of  the  light,  it  follows  that  in  an  isotropic  body  light 
is  propagated  in  all  directions  with  the  same  velocity. 

Crystals  of  the  other  systems  do  not  behave  in  this  manner. 
If  a  spot  of  light  is  observed  in  the  same  manner  as  before, 
through  a  transparent  cleavage  piece  of  calcite  it  appears  double. 
One  of  the  images  is  seen  in  the  true  position  and  the  other  is 
removed  from  it.  Depending  upon  the  position  of  the  calcite 
the  second  image  is  above  or  below,  to  the  right  or  to  the  left. 
If  the  calcite  be  rotated,  it  will  be  observed  that  the  latter 


FIG.  68. — Double  Refraction  in  Calcite. 


FIG.  69. — A  Principal  Section  in  Calcite. 


image  moves  in  a  circle  around  the  former.  It  may  also  be 
noted  that  the  line  joining  the  two  images  is  always  the  bisector 
of  the  obtuse  angle  of  the  cleavage  fragment.  All  crystals  that 
are  not  cubic,  act  in  a  similar  manner  and  are  therefore  called 
doubly  refracting  or  anisotropic. 

Double  Refraction  of  Light  in  Calcite. — Of  the  two  rays 
produced  by  the  double  refraction  of  calcite,  or  any  other 
hexagonal  or  tetragonal  crystal,  the  one,  w,  Fig..  68,  gives  an 
image  of  the  object  in  its  true  position,  i.e.  with  perpendicular 
incidence  of  the  light  it  suffers  no  deviation.  It  follows  the 
ordinary  laws  of  refraction  and  is  therefore  designated  as  the 
ordinary  ray.  The  other  ray,  e,  is  deviated  from  its  path. 
Different  laws  of  refraction  govern  it  and  it  is,  therefore,  called 
the  extraordinary  ray. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  53 

Any  plane  of  a  crystal  having  a  principal  crystallographic 
axis,  i.e.,  those  belonging  to  the  hexagonal  or  tetragonal  crystal 
systems,  is  called  a  principal  section,  provided  it  contains  the 
optic  axis.  Thus,  the  dotted  plane,  Fig.  69,  is  one  of  an  infinite 
number  of  principal  sections  containing  the  c  axis  of  calcite  and 
it  is,  indeed,  the  one  passing  through  two  edges  of  the  rhom- 
bohedron.  A  line  joining  the  two  images  produced  by  double 
refraction  in  every  section  of  calcite  corresponds  to  the  projec- 
tion of  a  principal  section  on  the  face  in  question.  From  this, 
it  follows  that  the  deviation  of  the  extraordinary  ray  in  calcite, 
or  in  any  hexagonal  or  tetragonal  crystal,  is  in  a  principal 
section.  In  a  cleavage  piece  of  calcite  the  line  joining  the  two 
images  produced  by  double  refraction  is  the  bisector  of  the 
obtuse  angle  of  the  calcite  rhombohedron,  Fig.  70. 


FIG.  70.  FIG.  71. 

Double  Refraction  in  a  Double  Refraction  in  two  parallel 

Rhomb  of  Calcite.  Rhombs  of  Calcite. 

If  the  images  seen  through  a  piece  of  calcite  are  observed 
through  a  second  cleavage  piece  of  the  same  thickness,  it  will  be 
found  that  they  do  not  behave  as  two  illuminated  openings. 
When  the  two  cleavage  pieces  are  exactly  parallel,  Fig.  71,  it  will 
be  noted  that  the  distance  between  the  two  images  is  doubled, 
i.e.,  the  double  refraction  of  one  piece  is  simply  added  to  that  of 
the  other.  If  one  calcite  is  rotated,  four  images  are  generally 
seen,  which  are  equally  bright  when  the  principal  sections  of 
the  calcite  form  an  angle  of  exactly  45°,  Fig.  72. 

The  significance  of  the  phenomenon  that  both  images  from  the 
first  calcite  did  not,  upon  rotation  of  the  second,  behave  as  two 
openings  illuminated  by  ordinary  light,  was  formerly  very 
difficult  to  understand.  An  explanation  was  only  possible 
after  the  difference  between  ordinary  and  polarized  light  was 
discovered.  The  former  consists  of  light  vibrating  in  all 
azimuths  at  right  angles  to  the.  direction  of  propagation,  while 
in  the  latter  the  vibrations  are  always  executed  in  but  one  plane, 


54  PETROGRAPHIC  METHODS 

which  is  likewise  perpendicular  to  the  direction  of  propagation, 
Fig.  22,  page  8.  It  is  a  noteworthy  fact  that  our  eye  is  not 
adjusted  to  discern  this  difference  between  ordinary  and  polarized 
light  and,  that  appliances  which  themselves  produce  polarization, 
are  necessary  to  recognize  it. 

The  phenomena  represented  in  Figs.  71-74  can  only  be 
explained  on  the  supposition  that  the  vibrations  of  light  in  the 
two  images  produced  by  the  first  calcite  are  polarized  in  two 
directions  at  right  angles  to  each  other;  and  further,  that  the 
vibration  directions  of  the  two  rays  are  perpendicular  to  each 
other  in  calcite,  the  one  taking  place  in  the  principal  section  and 
the  other  at  right  angles  to  it.  According  to  Fresnel's  hypoth- 
esis, which  is  here  taken  as  a  basis,  the  vibration  direction  of 
the  extraordinary  ray  is  always  in  the  principal  section  while  that 
of  the  ordinary  ray  is  perpendicular  to  it. 


FiG.JT2.  FIG.  73.  FIG.  74. 

<        Double  Refraction  in  two  Rhombs  of  Calcite  Rotated. 
45°  90°  180° 

The  phenomena  of  Figs.  71 — 74  can  be  explained  in  a  very 
simple  manner.  In  Fig.  71  the  light  which  emerges  from  the 
first  calcite  as  the  extraordinary  ray  e  passes  through  the  second 
parallel  to  it,  likewise  as  the  extraordinary  ray  e  and  suffers  the 
same  amount  of  deviation  in  the  second  as  in  the  first  and  in  fact 
in  the  same  direction.  The  other  ray  passes  through  both  calcites 
as  the  ordinary  rays  co  and  o  and  is  not  deviated.  If  the  second 
calcite  is  rotated  through  an  angle  of  45°,  Fig.  72,  the  ordinary  ray 
CD  of  the  first  is  resolved  by  the  second  into  two  components,  which 
correspond  to  the  two  vibration  directions  of  the  ordinary  ray 
o  and  the  extraordinary  ray  e  in  the  second  calcite.  Thus,  two 
images  are  formed,  of  which  one  coo  produced  by  a  light  passing 
through  both  calcites  as  an  ordinary  ray,  suffers  no  refraction. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  55 

The  other  cue  owes  its  origin  to  the  component  corresponding  to 
the  extraordinary  ray  of  the  second  calcite  and  is  therefore  re- 
fracted in  its  principal  section.  In  the  same  manner  the  extra- 
ordinary ray  of  the  first  calcite  e  gives  rise  to  two  images  eo 
and  ee,  which  are  shown  in  corresponding  positions  in  Fig.  72. 
In  a  position  in  which  the  two  principal  sections  enclose  an  angle 
less  than  45°,  the  two  images  coo  and  se  are  brighter  than  the 
other  two.  Upon  rotating  more  than  45°  the  two  latter  become 
brighter  at  the  expense  of  the  former  until,  after  a  rotation  of 
90°,  the  images  appear  as  shown  in  Fig.  73,  only  we  and  so  being 
visible.  When  at  last  the  calcite  has  been  rotated  through  180° 
the  conditions  are  as  in  Fig.  74.  The  ordinary  ray  of  the  first 
passes  as  an  ordinary  ray  through  the  second,  and  the  extraordi- 
nary ray  as  an  extraordinary.  The  former  is  not  deviated  at  all, 
while  the  latter  is  refracted  just  as  far  in  one  direction  in  the  first 
calcite  as  it  is  in  the  opposite  direction  in  the  second  calcite  so 
that  all  four  images  coincide. 

If  several  differently  orientated  plates  are  cut  from  a  crystal 
of  calcite,  two  images  of  an  illuminated  pinhole  can  generally  be 
seen  through  them,  but  the  distances  between  the  images  will 
vary  according  to  the  orientation  of  the  plates.  If  the  indices 
of  refraction  are  determined  in  these  plates  it  will  be  observed 
that  the  index  of  the  more  strongly  refracted  ray  is  the  same  in  all 
the  plates,  while  that  of  the  other  varies.  The  minimum 
value  of  the  latter  is  obtained  in  a  plate  cut  parallel  to  the  prin- 
cipal crystallographic  axis,  i.e.,  in  a  plate  in  which  the  light  pass- 
ing through  the  calcite  vibrates  parallel  and  perpendicular  to  that 
axis.  The  difference  between  the  velocities  of  the  two  rays  is 
greatest  when  the  light  is  propagated  perpendicular  to  the  prin- 
cipal crystallographic  axis.  Plates  parallel  to  the  base  give  but 
one  value  for  the  index  of  refraction.  In  this  case 'the  light  is 
propagated  parallel  to  the  principal  crystallographic  axis.  Hence 
it  follows  that  for  light  traveling  in  that  direction  calcite  behaves 
as  a  singly  refracting  substance  and  further  that  the  elasticity 
is  the  same  in  all  vibration  directions  perpendicular  to  the  prin- 
cipal axis. 

Uniaxial  Crystals. — That  direction  of  propagation  in  a  doubly 
refracting  crystal  in  which  light  suffers  no  double  refraction, 
i.e.,  perpendicular  to  which  all  vibration  directions  are  equal,  is 
called  the  optic  axis.  Calcite  and  all  other  hexagonal  and  tet- 
ragonal crystals,  i.e.,  all  crystals  with  a  principal  crystallographic 


56 


PETROGRAPHIC  METHODS 


axis,  possess  but  one  such  direction  and  are  therefore  called 
uniaxial.  The  ray  which  is  propagated  in  all  directions  with 
equal  velocity  is  the  ordinary  ray.  The  other  ray,  the  index  of 
refraction  and  the  velocity  of  propagation  of  which  are  depend- 
ent upon  the  direction  in  which  the  vibrations  take  place,  is 
the  extraordinary  ray.  It  follows  from  the  above  that  the  index 
of  refraction  or  its  reciprocal,  the  velocity  of  the  extraordinary 
ray,  more  nearly  approaches  that  of  the  ordinary  ray  the  smaller 
the  angle  between  the  optic  axis  and  the  direction  of  propaga- 
tion of  the  ray  through  the  crystal.  In  the  direction  of  the  optic 
axis  the  double  refraction  or  the  difference  between  the  indices 
disappears  because  all  vibrations  perpendicular  to  the  optic  axis 
have  equal  values. 

The  difference  between  the  indices  of  the  two  rays  passing 
through  a  uniaxial  crystal,  i.e.,  its  double  refraction,  is  greatest 
when  the  light  travels  through  the  crystal  perpendicular  to  its 
principal  crystallographic  axis,  which  is  also  the  optic  axis.  It 
becomes  zero  when  the  direction  of  propagation  coincides  with 


FIG.  75.  FIG.  76. 

Principal  Section  of  a  Wave  Surface  of  a  Uniaxial  Crystal. 

Positive.  Negative 


the  optic  axis  and  polarization  of  the  light  no  longer  takes  place. 
In  the  directions  lying  between  these  two,  the  double  refraction 
is  less  the  smaller  the  inclination  of  the  ray  of  light  to  the  optic 
axis. 

This  phenomenon  can  be  best  understood  by  considering  a 
wave  surface,  which  includes  all  points  to  which  light  would 
travel  from  the  center  of  a  crystal  in  a  given  unit  of  time.  Since 
the  portion  of  the  light  passing  through  as  an  ordinary  ray  vi- 
brates in  all  directions  with  equal  velocity  it  reaches  the  surface 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  57 

of  a  sphere  in  the  time  interval.  On  the  other  hand  the  extra- 
ordinary ray  possesses  different  velocities  in  different  directions 
and  its  velocity  is  equal  to  that  of  the  ordinary  ray  only  in  a 
direction  parallel  to  the  optic  axis.  Its  ray  surface  must  be  in 
contact  with  that  of  the  ordinary  ray  at  the  ends  of  the  axis. 
These  are  the  only  points  which  the  two  wave  surfaces  have  in 
common.  The  change  of  velocity  of  the  extraordinary  ray  is 
the  same  on  all  sides  for  equal  inclinations  of  the  direction  of 
propagation  to  the  optic  axis  and  increases  regularly  up  to  the 
maximum  perpendicular  to  the  optic  axis.  Therefore  the  wave 
surface  of  the  extraordinary  ray  must  be  a  rotational  form  differ- 
ent from  a  sphere.  It  is  an  ellipsoid  of  rotation,  Figs.  75  and  76. 
The  index  of  refraction  of  the  ordinary  ray  is  indicated  by 
cu,  and  that  of  the  ray  differing  from  it  the  most,  the  extra- 
ordinary ray,  by  e.  The  double  refraction  is  expressed  by  the 


FIG.  77. — Negative  FIG.  78. — Positive 

Double  Refraction. 

difference  e-co.  There  are  two  possible  cases;  one,  as  in  calcite, 
where  oj  >  e  that  is,  the  ordinary  ray  is  more  strongly  refracted 
than  the  extraordinary.  Then  e-co  is  negative  and  such  crystals  are 
said  to  be  negative  or  because  the  extraordinary  ray  is  farther  from 
the  normal  than  the  ordinary  ray,  they  are  also  called  repellant 
crystals,  Fig.  77.  In  the  other  case  £  >a>,  the  crystal  -is  said  to  be 
positive  or  attractive,  Fig.  78.  Since  the  velocity  is  the  reciprocal 
of  the  index  of  refraction,  the  extraordinary  ray  of  a  negative 
crystal  and  the  ordinary  ray  of  a  positive  crystal  have  the  greatest 
velocities.  The  direction  of  greatest  elasticity  and  least  index 
of  refraction  is  called  tt1  and  the  corresponding  index  a;  the 
direction  of  least  elasticity  c  and  its  index  ?-,  while  c  is  the  prin- 
cipal crystallographic  axis.  c  =  a  is  the  symbol  of  a  negative 

xUsage  differs  somewhat.  In  America  X,  Y,  Z  is  proposed  by  Iddings  for  0,  B,  t  re- 
spectively, while  in  France  Up  (p  =  petit,  small)  ng  (g  =  grand,  large)  and  nm  (m  = 
mo  yen,  medium)  are  used  for  CLfft  respectively,  b  and  /3  refer  of  course  to  biaxial 
crystals. 


58  PETROGRAPHIG  METHODS 

crystal,  c  =  t  that  of  a  positive,  while  f-a.  shows  the  amount  of 
double  refraction  in  all  cases. 

Biaxial  Crystals. — The  conditions  are  more  complex  in  crystals 
of  the  orthorhombic,  monoclinic,  and  triclinic  systems.  If  an  il- 
luminated opening  is  observed  through  several  plates  of  aragonite 
variously  orientated,  it  will  be  noted  that,  in  general,  two  images 
are  seen,  but  both  appear  to  be  displaced  from  their  original 
positions.  In  this  case  the  light  is  resolved  into  two  extraordinary 
rays  which  are  always  polarized  at  right  angles  to  each  other. 
If  the  indices  of  refraction  are  determined  with  a  series  of 
prisms  of  aragonite  it  will  be  noted  that  none  of  them  is  con- 
stant. The  maximum  and  minimum,  i.e.,  the  greatest  double 
refraction,  is  observed  in  a  prism  in  which  the  direction  of  pro- 
pagation of  the  ray  is  parallel  to  the  crystallographic  a  axis. 
The  two  rays  vibrate  at  right  angles  to  each  other  and  parallel 
to  the  other  two  crystallographic  axes,  which  are  at  the  same  time 
the  directions  of  greatest  and  least  velocity  of  light  or  the  direc- 
tions of  smallest  and  greatest  index  of  refraction  in  the  crystal. 
In  all  other  directions  the  indices  of  refractron  are  intermediate 
between  these  two  extremes,  including  also  light  vibrating  in  the 
direction  of  the  a  axis.  Three  optical  directions  perpendicular 
to  each  other  must  be  distinguished  in  aragonite  and  these  coin- 
cide with  the  a,  b,  and  c  axes.  On  the  other  hand,  there  is  no 
relationship  existing  between  the  value  of  a  vibration  direction 
whether  a,  b,  or  c  and  the  crystallographic  axis  because  in 
the  orthorhombic  system  the  crystallographic  axes  can  be  placed 
in  any  position  desired.  The  three  principal  vibration  directions 
of  light,  the  axes  of  optical  elasticity  in  a  biaxial  crystal,  can  be 
designated  in  various  ways  depending  upon  whether  the  index 
of  refraction  or  its  reciprocal  value,  the  velocity  of  light,  is  to  be 
emphasized. 


f  smallest  a 

Index  of  refraction     <  medium  ft 
[  largest     7- 


a  =  1  :  a  largest.     ]    Velocity  of 


B  =  1  :  /?  medium     [         light. 
t  =  \if  smallest    j    Elasticity. 


Let  us  now  consider  more  in  detail  the  bundle  of  planes,  the 
axis  of  which  is  the  direction  of  medium  index,  /?.  Light  passing 
through  the  mineral  perpendicular  to  one  of  these  planes  is 
resolved  into  two  rays,  one  of  which  always  vibrates  in  the 
direction  of  medium  velocity  and  the  other  is  perpendicular  to 
it.  Its  vibration  direction  coincides  either  with  that  of  the 
largest  (/)  or  of  the  smallest  (a)  index  or  it  corresponds  to  a 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHTS 

medium  value  between  these  two.  Among  these  values  there 
is  one  /?',  Fig.  79,  equal  to  jt.  For  the  direction  of  propagation 
AA,  which  lies  in  the  plane  a  f  and  perpendicular  to  the  plane 
$  /?',  there  are  two  perpendicular  vibration  directions  of  equal 
value.  A  A  is  an  optic  axis.  Since  the  plane  a  ft  in  aragonite 
is  a  plane  of  symmetry,  another  direction  equal  to  AA  and 
symmetrical  to  it,  must  lie  on  the  other  side  of  a  in  the  plane 
a  fj  and  a  and  y  will  bisect  the  angle  between  these  two  optic 
axes.  Another  such  direction  is  crystallographically  impossible 
and  therefore  aragonite,  and  with  it  all  crystals  of  the  ortho- 
rhombic,  monoclinic,  and  triclinic  systems,  are  biaxial. 

The  plane  in  which  the  two  optic  axes  lie  is  called  the  plane 
of  the  optic  axes.     The  directions  of  largest  (f)  and  smallest  (a) 


cc    A 


A    a 
FIG.  79. — Construction  of  an  Optic  Axis  in  Aragonite. 


index  bisect  the  angles  of  the  optic  axes  and  are  called  middle 
lines  or  bisectrices.  The  direction  of  medium  index  '(/?),  perpen- 
dicular to  the  plane  of  the  optic  axis,  is  called  the  optic  normal. 
The  optic  axes  form  an  acute  angle  on  one  side  and  an  obtuse 
on  the  other.  The  bisector  of  the  acute  angle  is  called  the  first 
middle  line  or  the  acute  bisectrix  Bxa  and  the  bisector  of  the 
obtuse  angle,  the  second  middle  line  or  obtuse  bisectrix  Bx0. 
If  the  acute  bisectrix  is  the  direction  of  the  smallest  index  (a) 
the  crystal  is  said  to  be  negative  and  in  the  other  case  positive. 
The  value  of  the  intermediate  index  can  be  the  arithmetic  mean 
of  the  largest  and  smallest  indices  for  a  certain  color  only  when 
the  optic  angle  for  that  color  is  90°,  otherwise  it  is  the  nearer 


60  PETROGRAPHIC  METHODS 

that  of  the  obtuse  bisectrix  the  smaller  the  acute  optic  angle, 
and  when  it  becomes  0°  the  two  directions  are  equivalent  and 
the  crystal  is  optically  uniaxial. 

Since  the  index  of  refraction  is  different  for  different  colors 
in  one  and  the  same  crystal,  the  size  of  the  optic  angle  must 
vary  with  the  color.  Sometimes  the  acute  angle  for  red  (p)  is 
larger  than  that  for  violet  (u),  which  is  indicated  by  the  symbol 
of  dispersion  p  >  u.  The  opposite  o  >  p  is  also  encountered. 

As  seen  in  the  case  of  aragonite,  the  plane  of  the  optic  axes 
in  orthorhombic  crystals  is  always  parallel  to  one  of  the  three 
pinacoids  because  the  directions  of  greatest  and  least  elasticity 
coincide  with  two  of  the  crystallographic  axes.  In  the  mono- 
clinic  system  only  one  of  the  three  principal  vibration  directions 
coincides  with  the  b  axis  and  this  is  the  case  for  all  colors.  The 
plane  of  the  optic  axes  lies  parallel  or  perpendicular  to  the 
clinopinacoid  and  the  other  two  vibration  directions  fall  in  the 
plane  of  symmetry,  wherever  they  may,  but  are  always  per- 
pendicular to  each  other.  They  may  have  quite  different 
positions  in  that  plane  for  different  colors,  thus  causing  dis- 
persion of  the  bisectrices  or  the  planes  of  the  optic  axes.  In  the 
triclinic  system  the  optical  elements  are  orientated  entirely 
independently  of  the  crystallographic  axes  and  likewise  the 
relationships  for  the  different  colors  are  independent  of  each 
other.  Even  here,  however,  the  three  principal  vibration 
directions  for  any  one  color  are  perpendicular  to  each  other. 

The  velocity  of  light  in  any  direction  in  a  biaxial  crystal 
can  be  easily  shown  by  the  Fresnel  principle.  An  ellipsoid  is 
described  about  the  three  axes  of  elasticity  of  a  biaxial  crystal 
and  a  plane  is  placed  in  the  center  perpendicular  to  the  direction 
of  propagation  of  the  ray.  The  form  of  such  a  section  is  always 
an  ellipse,  except  in  the  two  positions  perpendicular  to  the 
optic  axes,  where  it  is  a  circle.  The  lengths  of  the  large  and 
small  axes  of  the  elliptical  section  correspond  to  the  velocities 
of  the  two  rays  propagated  perpendicular  to  the  section. 

2.  Investigations  with  One  Nicol — the  Polarizer 

Surface  Color  and  Pleochroism. — When  light  enters  an  aniso- 
tropic  crystal  it  is  generally  resolved  into  two  rays  propagated 
with  different  velocities.  These  rays  are  absorbed  differently 
and,  when  this  difference  is  great  enough,  variation  in  color  may 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  61 

be  noted.  If  a  doubly  refracting  crystal  is  illuminated  with 
ordinary  light,  the  mean  of  the  two  vibrations  is  observed  at 
any  moment  because,  as  a  result  of  the  rapid  change  of  the 
planes  of  vibration  in  a  ray  of  ordinary  light,  equal  amounts 
of  light  pass  through  the  crystal  at  a  given  instant  parallel  to 
each  of  the  vibration  directions.  Upon  looking  through 
a  surface  of  a  crystal  in  ordinary  light  a  mixed  color  is  observed, 
which  is  called  the  surface  color.  If,  on  the  other  hand,  plane 
polarized  light  enters  a  doubly  refracting  crystal,  the  two  com- 
ponents corresponding  to  the  vibration  directions  in  the  crystal 
will  be  alike  only  when  they  form  an  angle  of  45°  with  the 
vibration  directions  of  the  nicol.  One  direction  will  become 
zero,  i.e.,  only  those  vibrations  parallel  to  the  other  direction 
will  be  observed,  when  the  vibration  direction  of  the  nicol 
coincides  with  it. 

The  properties  of  each  ray  passing  through  a  crystal  can  be 
studied  in  succession  by  placing  the  vibration  directions  parallel 
to  that  of  the  polarizer  between  crossed  nicols  and  then  removing 
the  analyzer  to  observe  the  color.  Thus  the  color  for  certain 
directions  and  the  differences  for  different  directions  of  vibration 
can  be  observed.  These  colors  are  called  the  colors  of  the  axes. 
Variation  in  color  for  different  directions  in  a  crystal  is  called 
pleochroism. 

For  these  investigations  the  polarizer  beneath  the  preparation  is  used, 
while  the  analyzer  is  pushed  out  of  the  tube.  If  the  analyzer  were  used 
the  interference  phenomena,  caused  by  the  partially  polarized  light  reflected 

from  the  mirror,  although  not  very  dis- 
tinct, would  greatly  interfere  with  the 

[L  y=  ^i|j!!!!|     results.      If  the  colors  parallel  to  the 

rri— >  ^\  K  \     Ili       axes  are  to  be  observed  side  by  side,  in- 

stead of  one  after  the  other,  a  Haidinger 
lens,  Fig.  80,  is  used.      This  consists  of 
a  tube  with  a  rectangular  opening  in 
FIG.  80.— Haidinger  Lens.  One    end    and    contains    an    elongated 

piece    of    calcite    K,    which   is   of   the 

proper  length  to  give  two  images  of  the  opening  side  by  side.  The  vibra- 
tion directions  of  the  polarized  light  are  perpendicular  to  each  other  in 
these  two  images  produced  by  double  refraction.  If  the  crystal  in  front 
of  the  opening  is  rotated  until  the  two  images  show  the  greatest  possible 
difference  in  color,  the  two  principal  colors  of  the  face  are  observed.  A 
similar  device  is  the  dichroscopic  ocular,  which  may  be  used  for  observing 
the  two  principal  colors  under  the  microscope.  Here,  however,  the  results 
are  disturbed  by  the  partial  polarization  of  light  reflected  from  the  mirror, 
as  indicated  above. 


62  PETROGRAPHIC  METHODS 

Cross  sections  of  cubic  crystals  or  of  amorphous  substances 
always  show  the  same  color  upon  a  complete  rotation  of  the  stage 
above  the  polarizer  because  in  them  the  absorption  of  light  is 
the  same  in  all  directions.  In  uniaxial  minerals  all  directions 
perpendicular  to  the  optic  axis  are  of  equal  value  in  an  optical 
sense.  Only  the  color  of  the  ordinary  ray  is  observed  on  the  base 
of  a  uniaxial  crystal  in  ordinary  light  and  this  is  the  same  as 
the  surface  color.  Such  a  section  does  not  change  its  color  upon 
rotation  of  the  stage.  The  absorption  of  light  is  the  same  for 
the  ordinary  ray  in  any  section  and  its  color  can  be  observed 
in  any  direction,  while  the  absorption  of  the  extraordinary  ray 
is  variable  and  differs  the  more  from  that  of  the  ordinary  ray 
the  nearer  its  vibration  direction  approaches  the  direction  of 
the  optic  axis.  The  color  of  the  ordinary  ray  can  be  observed 
in  any  section,  while  that  of  the  extraordinary  ray  appears  only 
in  a  section  parallel  to  the  optic  axis  of  the  crystal.  If  the 
difference  in  absorption  for  the  two  principal  directions  of  an 
uniaxial  crystal  is  sufficiently  large  to  be  observed  it  will  be  seen 
most  perfectly  in  a  section  parallel  to  the  optic  axis.  Since  in 
the  case  of  uniaxial  crystals  there  are  only  two  principal  colors, 
it  is  common  to  speak  of  dichroism. 

In  biaxial  crystals  there  are  three  different  vibration  directions 
for  color  as  well  as  for  indices  of  refraction.  These  crystals  possess 
three  principal  colors  and  are  termed  trichroic.  The  pure  color 
belonging  to  one  of  these  vibration  directions  can  be  observed 
only  in  a  section  parallel  to  it.  In  all  others  mixed  colors  are 
seen.  In  the  orthorhombic  system  the  vibration  directions  of 
the  greatest,  least  and  intermediate  indices  of  refraction  are  at. 
the  same  time  the  color  axis.  In  the  monoclinic  system  only  the 
b  axis  is  simultaneously  a  color  axes.  The  other  two  lie  anywhere 
in  the  plane  of  symmetry,  but  are  not  very  far  removed  from  the 
vibration  directions  of  light.  In  the  triclinic  system  all  these 
directions  are  independent  of  each  other  but  even  in  this  case, 
if  a  vibration  direction  is  placed  parallel  to  the  polarizer  instead 
of  a  color  axis,  the  error  is  scarcely  noticeable. 

The  Phenomenon  of  Pleochroism. — The  difference  of  absorption  of  light 
in  various  directions,  which  is  known  as  pleochroism,  is  so  slight  in  numerous 
cases,  especially  in  colorless  crystals,  that  it  cannot  be  observed  even  with 
thick  plates  of  the  mineral.  Since  the  color  of  a  section,  and  likewise  the 
difference  of  color  in  different  directions,  is  less  distinct  the  thinner  the 
layer,  it  follows  that  in  thin  section,  pleochroism  can  only  be  observed  in 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  63 

substances  having  a  great  difference  of  absorption  in  different  directions. 
Although  in  the  observation  of  weak  pleochroism  the  subjective  ideas  of  the 
observer  and  a  certain  amount  of  practice  are  important,  still  the  investi- 
gation of  pleochroism  is  a  valuable  aid  in  microscopic  research. 

It  is  very  useful  in  the  study  of  pleochroism  to  keep  three  possible  cases 
in  mind.  This  can  be  done  best  by  the  aid  of  a  color  scale  such  as  a  Radde 
color  scale,  which  shows  the  different  colors  in  a  horizontal  row  and  the 
different  intensities  of  each  from  the  darkest  to  the  lightest  tints  in  vertical 
columns.  The  following  cases  are  possible : 

1.  Different  parts  of  the  spectrum  are  absorbed  in  different  parts  of  the 
crystal  in  such  a  manner  that  the  intensity  of  the  light  which  travels  in 
these  directions  remains  approximately  the  same.     Various  colors  appear 
in  different  directions,  but  one  is  about  as  bright  as  the  other.     Compare 
horizontal  series  of  the  color  scale. 

2.  In  each  direction  approximately  the  same  portions  of  the  spectrum  are 
absorbed  but  with  unequal  intensity.     The  same  color  is  observed  in  all 
directions,  but  with  different  intensities.     Compare  vertical  series  of  the 
color  scale. 

3.  In  different  directions  different  colors  of  the  spectrum  are  absorbed 
with  different  intensities.     The  different  colors  are  seen  in  different  shades. 
Compare  the  diagonal  of  the  color  scale. 

These  three  cases  often  give  rise  to  very  characteristic  phenomena,  e.  g., 
pleochroic  pyroxene  almost  always  shows  the  same  depth  of  color,  in  one 
direction  light  green,  in  the  other  light  yellow.  Common  hornblende  of 
the  same  color  shows  stronger  absorption  of  the  green  ray  and  its  pleo- 
chroism is  from  dark  green  to  light  yellow,  while  in  biotite  only  simple 
differences  of  absorption  of  the  same  color  are  effective  and  it  changes  from 
light  brown  to  dark  brown. 

The  strength  of  pleochroism  in  one  and  the  same  crystal  is  dependent 
upon  its  thickness  and  very  frequently  the  phenomenon  can  be  observed  in 
thicker  layers  when  it  is  not  noticed  at  all  in  thin  sections.  The  absorp- 
tion is  rarely  so  great  that  no  light  is  observed  passing  through  the  crystal 
in  that  direction.  The  color  of  the  more  strongly  absorbed  ray  can  almost 
always  be  recognized,  even  in  the  most  strongly  absorbing  minerals  if  the 
section  is  thin  enough. 

The  orientation  of  the  differently  absorbed  rays  in  a  crystal  is  of  import- 
ance because  it  tends  to  be  strikingly  constant  in  one  and  the  same  sub- 
stance, as  also  in  the  larger  isomorphic  series.  Two  points  of  view  are  to 
be  considered  in  practice:  1.  The  relationship  of  the  more  strongly 
absorbed  ray  to  the  crystallographic  development  of  the  substance  in 
question,  i.  e.,  to  the  long  direction  of  its  cross  section,  the  principal  zone. 
Almost  all  important  rock-forming  minerals  with  great  differences  of 
absorption  show  stronger  absorption  of  the  ray  vibrating  parallel  or  nearly 
parallel  to  the  principal  zone.  They  have  a  deeper  color  when  the  longer 
edge  of  the  cross  section  is  parallel  to  the  vibration  direction  of  the  polarizer. 
Fig.  81.  Tourmaline  behaves  in  the  opposite  manner  and  shows  a  lighter 
color,  Fig.  82.  Stronger  absorption  of  light  in  the  direction  perpendicular 
to  the  principal  zone  is  one  of  the  most  important  means  for  determining 
this  mineral,  which  often  occurs  in  minute  individuals  that  are  difficult  to 


64 


PETROGRAPHIC  METHODS 


determine  by  other  optical  properties.  2.  The  relation  between  the 
refraction  and  absorption  of  the  two  rays  vibrating  in  a  section.  There  is 
no  regular  connection  between  these  two  properties  and  although  in  most 
cases  in  rock-forming  minerals  the  most  strongly  refracted  ray  is  the  most 
strongly  absorbed,  this  is  by  no  means  always  the  case.  In  the  above 
mentioned  example,  tourmaline,  the  extraordinary  ray  is  the  least  refracted 
and  the  least  absorbed.  Its  absorption  symbol  is  ©  >o  or  since  tourmaline 
is  optically  negative  w>£.  In  apatite,  which  is  likewise  negative,  the 
more  strongly  refracted  ray  is  orientated  in  reverse  manner.  Here  the 
symbol  is  o>©  or  £>&).  Similar  results  are  encountered  in  optically 
biaxial  minerals.  Common  hornblende,  for  example,  universally  shows 
c>6  >  a,  while  with  riebeckite  and  arfvedsonite,  on  the  other  hand,  o  >B  >©. 

Pleochroism  can  be  produced  in  an  artificial  manner  under  certain 
conditions  and  may  become  a  very  good  characteristic  of  a  substance. 
When  members  of  the  olivine  or  amphibole  groups  containing  iron  are 
roasted,  they  assume  strong  colors  and  decided  differences  of  absorption. 
This  serves  to  distinguish  them  from  the  pyroxenes  which  generally  do  not 


FIG.  81. — Stronger  absorption  Parallel 
to  Principal  Zone. 


FIG.  82. — Stronger  Absorption  Perpen- 
dicular to  Principal  Zone. 


show  any  pleochroism  as  the  result  of  such  operations.  Fibrous  and 
scaly  mineral  aggregates  can  be  colored  directly  with  aniline  dyes,  where- 
upon many  of  them  such  as  talc  and  serpentine  become  distinctly  pleochroic. 
Many  artificial  crystals,  colorless  in  themselves,  acquire  not  only  color  but 
often  distinct  pleochroism  upon  crystallization  from  a  dye  solution.  This 
phenomenon  has  been  known  a  long  time  in  the  case  of  strontium  nitrate. 
For  the  sake  of  completeness  pleochroic  halos,  Figs.  83  and  84,  will  be 
considered  briefly.  In  numerous  minerals,  particularly  in  cordierite, 
andalusite,  mica,  hornblende,  tourmaline,  etc.,  it  will  be  noticed  that  certain 
rounded  spots  show  stronger  pleochroism  than  the  rest  of  the  mineral 
generally  does.  If  these  portions  are  studied  more  carefully  small  inclusions 
of  foreign  minerals  will  be  discovered  in  the  center  of  such  pleochroic  halos 
and  the  outline  is  surrounded  on  all  sides  by  a  more  pleochroic  zone. 
Since  it  is  possible  to  produce  phenomena  analogous  to  the  pleochroism  in 
these  minerals  by  illuminating  with  radium  preparations  and,  further  since 
most  of  the  inclusions  forming  the  centers  of  such  halos — zircon,  rutile, 
orthite,  etc. — are  radio-active,  there  can  be  no  doubt  that  the  pleochroism 
of  the  halo  depends,  for  the  most  part  at  least,  upon  the  action  of  active 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT 65 


rays.  It  is  noteworthy  that  refraction,  double  refraction,  and  frequently 
also  dispersion  are  greatly  changed  in  the  pleochroic  halos.  They  are 
sometimes  decidedly  lower,  sometimes  much  higher.  Besides  the  radio- 
active inclusions  surrounded  by  pleochroic  halos  there  are  frequently  other 
crystalline  inclusions  in  the  same  crystal,  which  are  not  classed  with  radio- 
active minerals  (and  do  not  show  this  phenomenon),  for  example,  the  long 
apatite  needles  in  Fig.  84. 

Two  other  phenomena  which  are  observed  now  and  then  must  also  be 
mentioned.  One  of  these  is  pseudodichroism,  a  phenomenon  which  occurs 
in  perfectly  colorless  crystals.  If  there  are  a  large  number 
of  parallel  orientated  inclusions  in  a  crystal  and  the  refrac- 
tion of  these  differs  considerably  from  that  of  the  surround- 
ing crystal,  light  passing  through  is  disturbed  by  the  inclu- 
sions if  they  are  steeply  inclined  to  the  direction  of  propa- 
gation of  the  light.  The  less  refracted  red  rays  are  bent 
toward  the  middle  and  the  more  strongly  refracted  violet 
rays  toward  the  edge  of  the  field  of  vision.  Only  those 
rays  vibrating  par- 
allel to  the  direction 
of  the  inclusions  suffer 
this  sort  of  refraction, 
while  those  vibrating 
at  right  angles  to  it 
are  not  changed.  If 
the  crystal  is  so  orien- 
tated in  the  center  of 

the  field  that  the  direction  of  the  inclusions  is  parallel  to  the  principal 
direction  of  the  polarizer,  it  appears  a  brilliant  brown.  If  it  is  on  the  edge, 
it  is  colored  gray,  but  in  each  case  it  becomes  colorless  upon  rotating 
through  90°. 

The  occurrence  of  interference  colors  without  the  use  of  the  analyzer  is 
the  other  of  the  phenomena  referred  to  above.  If  a  layer  of  a  doubly 
refracting  mineral,  orientated  at  random  occurs  in  a  slide  under  a  thin  crystal 
of  a  very  strongly  absorbing  mineral — biotite  or  tourmaline — the  absorb- 
ing crystal,  which  scarcely  allows  one  of  the  rays  to  pass  through,  acts  as  an 
analyzer  itself  and  the  mineral  beneath  it  shows  interference  colors.  Com- 
plementary colors  appear  upon  a  rotation  through  90°,  because  the  absorb- 
ing direction  of  the  preparation  is  first  perpendicular  and  then  parallel  to 
the  principal  direction  of  the  polarizer.  The  occurrence  of  interference 
colors  in  minerals  with  a  high  double  refraction — calcite  and  rutile — which 
are  crossed  by  twininng  lamellae  greatly  inclined  to  the  face  of  the  slide,  is  a 
similar  phenomenon. 

3.  Investigations  with  Crossed  Nicols 

Observations  between  two  nicols  are  carried  out  almost 
entirely  with  the  planes  of  vibration  of  the  two  crossed,  and  for 
this  reason  they  are  called  observations  with  crossed  nicols 


FIG.  83.  FIG.  84. 

Pleochroic  Halos  around  Inclusions  of  Zircon  in 
Tourmaline.  Biotite. 


66 


PETROGRAPHIC  METHODS 


The  following  determinations  are  to  be  made  in  parallel  polarized 
light  with  the  microscope,  as  commonly  constructed,  which 
affords  an  enlarged  image  of  the  object  investigated: 

(a)  Recognition  of  double  refraction. 

(b)  Determination  of  the  position  of  the  vibration  directions. 

(c)  Measurement  of  the  strength  of  double  refraction. 

(d)  Determination  of  the  relative  velocities  of  the  two  rays 
vibrating  in  a  cross  section. 

Recognition    of   Double    Refraction. — If    a    singly  ^refracting 
substance,   or  a  section  through  such  a  substance,   is  placed 
between  crossed  nicols,  the  plane  polarized  light  from  the  polar- 
izer passes  through  the  crystal  unaltered  to  the  analyzer,  and 
since  its  vibration  direction  is  at  90°  to  that 
of    the   approaching   ray  no   light  will   pass 
through.      No  change  occurs  upon  rotating 
"Q/    the  stage'  through  360°.     A  singly  refracting 
substance   appears  dark  in  all  positions  be- 
tween crossed  nicols. 

If  a  doubly  refracting  crystal  is  observed 
between  crossed  nicols,  the  light  coming  from 
the  polarizer  suffers  no  change  in  the  crystal 
only  when  the  vibration  directions  RR'  and 
SS'  in  the  crystal  are  exactly  parallel  to  the 
vibration  directions  PP'  and  QQ'  in  the  two 
nicols,  Fig.  85.  If  the  vibration  directions 
RR'  and  SS'  are  oblique  to  PP7  and  QQ',  as  in 
Figs.  86  to  88,  the  plane  polarized  light  from 
the  polarizer  is  resolved  into  two  components 
Or  and  Os'  corresponding  to  the  vibration 
FIG.  85.— vibration  directions  of  the  crystal.  After  another 
Directions  in  Crystal  resolution  in  the  analyzer  the  components 

Parallel  to  Those  of  the  .  v     •,.".' v\  J     /^* 

Nicols.  passing    through   it,   Op   and    Oo  are    com- 

bined. The  plate  appears  illuminated  in  all 
other'  positions  because  a  portion  of  the  light  always  passes 
through  the  analyzer.  The  magnitude  of  the  components  Op 
and  OS  indicates  the  brightness  and  it  follows  from  a  com- 
parison of  Figs.  86  to  88  that  the  maximum  is  reached  when 
the  vibration  directions  of  the  crystal  are  at  45°  to  those  of  the 
nicols,  Fig.  87. 

Since  the  vibration  directions  RR'  and  SS'  are  parallel  to  PP' 
and  QQ'  four  times  in  a  horizontal  rotation  of  the  stage  through 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  67 

360°,  doubly  refracting  crystals  are  alternately  light  and  dark 
four  times  in  a  rotation  of  360°  between  crossed  nicols.  Maxi- 
mum brightness  is  obtained  when  the  vibration  directions  of  the 
crystal  are  at  45°  to  those  of  the  nicols.  It  diminishes  upon 
further  rotation  and  passes  gradually  over  into  complete  darkness 
when  these  directions  are  respectively  parallel.  This  latter 
position  is  also  called  the  position  of  extinction,  and  the  vibration 
directions  in  the  crystal,  the  extinction  directions.  The  determi- 
nation of  the  latter  is  an  important  aid  in  many  cases  for  ascer- 
taining the  crystal  system. 


-Q'      Q 


'     Q- 


p' 

FIG.  87. 
Vibration  Directions  in  Crystal  Oblique  to  Those  of  the  Nicols. 

Gray  interference  colors  of  the  lowest  order  (see  Fig.  96,  page  75),  corre- 
sponding to  the  lowest  degrees  of  double  refraction,  are  extremely  difficult 
to  distinguish  from  total  darkness.  Therefore,  when  thin  sections  of  very 
weak  double  refracting  crystals  are  examined,  it  is  sometimes  difficult  to 
distinguish  whether  or  not,  there  is  any  effect  on  polarized  light  at  all.  To 
recognize  very  weak  double  refraction,  use  is  made  of  certain  accessory 
devices — sensitive  tint  plate,  Bravais  double  plate,  etc. — which  will  be 
described  more  in  detail  under  the  head  of  stauroscopes.  The  change  of 
interference  color  in  such  devices,  caused  by  even  extremely  low  double 
refracting  plates,  is  much  more  easily  determined  than  the  difference  be- 
tween absolute  darkness  and  very  weak  illumination,  so  that  the  presence 
of  double  refraction  can  be  definitely  determined,  even  in  the  most  doubtful 
cases. 

Determination  of  the  Position  of  the  Vibration  Directions. 

The  position  of  the  vibration  directions  can  be  determined 
from  the  above  by  placing  the  crystal  between  crossed  nicols  in 
the  position  of  perfect  darkness.  Then  the  vibration  directions 
of  the  crystal  lie  parallel  to  those  of  the  two  nicols,  and  the  latter 
are  exactly  parallel  to  the  cross  hairs  in  an  adjusted  microscope. 


68 


PETROGRAPHIC  METHODS 


A  characteristic  crystal  edge  or  cleavage  direction  is  brought 
parallel  to  one  of  these  hairs  by  rotating  the  stage,  and  a  reading 
is  made  on  the  scale  on  the  stage.  Then  the  analyzer  is  inserted 
and  the  stage  rotated  until  the  crystal  appears  perfectly  dark, 
and  another  reading  of  the  scale  is  made,  the  difference  being 
the  angle  which  the  vibration  direction  makes  with  the  edge  or 
cleavage  in  question. 

The  position  of  absolute  darkness  may  not  be  found  accurately 
on  account  of  the  gradual  transition  from  light  to  dark.  The 
beginner  will  experience  considerable  difficulty  in  finding  the 
true  position  of  darkness.  The  method  of  procedure  is  as 
represented  in  Fig.  89.  PP'  and  QQ'  represent  the  vibration 


FIG.  89. — Determination  of  Extinction  Direction. 

directions  of  the  two  nicols.  The  stage  is  rotated  in  the  direction 
of  the  arrow  I  until  the  crystal  appears  entirely  dark  and  its 
long  edge,  which  may  formerly  have  been  parallel  to  PP',  has 
reached  the  position  A'.  Then  it  is  rotated  further  toward  QQ', 
and  the  crystal  becomes  light  again.  It  is  then  rotated  back  in 
the  direction  of  the  arrow  II  until  darkness  is  obtained,  say  in 
the  position  A".  The  direction  a,  which  bisects  the  angle  between 
the  two  directions  a'  and  a",  i.e.,  the  mean  of  the  two  positions,  is 
the  true  vibration  direction.  If  this  operation  is  repeated 
several  times,  the  mean  of  a  large  number  of  readings  will  come 
very  near  to  the  true  position  of  darkness.  Even  though  the 
greatest  care  is  exercised,  an  error  of  ±  J°,  as  is  generally  the 
case  with  microscopic  measurements,  can  scarcely  be  avoided. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHTS 

In  general  parallel  or  symmetrical  extinction,  Figs.  90  and  91,  is  observed 
in  hexagonal,  tetragonal  and  orthorhombic  crystals,  because  the  vibration 
directions  coincide  with  the  crystal  axes.  The  determinations  are  made 
in  white  light  because  the  vibration  directions  are  the  same  for  all  colors. 
Monoclinic  crystals  show  parallel  or  symmetrical  extinction  only  upon 
faces  in  the  zone  of  the  6  axis,  and  for  all  colors,  while  on  all  other  faces  the 
extinction  is  oblique,  Fig.  92,  i.e.,  the  extinction  directions  are  unsymmet- 
rical  with  respect  to  the  crystal  edges.  Since  this  variation  is  different 
for  different  colors,  the  position  of  absolute  darkness  cannot  be  obtained 
with  white  light.  The  extinction  is  best  determined  in  monochromatic 
light  if  an  accurate  measurement,  and  not  merely  an  approximate  orienta- 
tion is  to  be  made. 


FIG.  90.— Parallel 


FIG.  91. — Symmetrical. 
Extinction. 


FIG.  92.— Oblique 


When  the  extinction  angle  is  determined  it  must  be  noted  whether  the 
measurement  from  the  crystal  edge  is  to  the  front  or  to  the  rear.  The 
difference  caused  by  confusion  of  these  two  directions  is  shown  in  Fig.  92. 
The  dotted  arrow  shows  the  false  vibration  direction,  the  proper  direction 
of  which  is  given  by  the  solid  arrow.  A  beginner  will  do  well  to  sketch 
the  crystal  and  place  the  sketch  parallel  to  the  extinguished  crystal,  and 
then  draw  lines  parallel  to  the  vibration  directions,  i.e.,  parallel  to  the  cross 
hairs.  This  shows  the  vibration  directions  sought  in  their  true  positions. 

As  mentioned  above,  hexagonal,  tetragonal  and  orthorhombic 
crystals  generally  show  parallel,  extinction  because  their  develop- 
ment is  usually  parallel  or  symmetrical  to  the  principal  vibration 
directions.  Monoclinic  crystals  are  not  uncommonly  developed 
so  as  to  be  long  prismatic  parallel  to  the  b  axis,  e.g.,  epidote  and 
wollastonite,  and  then  the  principal  zone  is  the  position  of 
parallel  extinction.  It  is  obvious  that  crystals  thus  developed 
may  be  confounded  in  thin  section  with  those  showing  parallel 
extinction  in  all  sections,  because  cross  sections  transverse  to  the 
direction  of  elongation  are  not  very  numerous.  The  true  charac- 
ter of  monoclinic  crystals  can,  however,  be  easily  determined  if 
they  are  developed  tabular  parallel  to  the  base  or  prismatic 


70 


PETROGRAPHIC  METHODS 


parallel  to  one  of  the  directions  lying  in  the  plane  of  symmetry. 
Triclinic  crystals  may  be  developed  in  various  ways  and  can 
rarely  be  orientated  properly. 

The  value  of  the  extinction  angle  of  monocline  crystals  is 
determined  on  the  clinopinacoid  and  where  there  is  no  statement 
to  the  contrary,  this  face  is  always  understood.  Other  faces  can 
be  used  for  the  determination  of  a  crystal  only  when  their  ex- 
tinction directions  are  accurately  known.  The  directions  can- 
not generally  be  determined  from  the  inclination  of  the  face  to 
the  b  axis,  because  the  extinction  angle  does  not  diminish  in  a 
regular  manner  in  passing  from  the  clinopinacoid  to  the  ortho- 
pinacoid,  and  may  even  have  a  higher  value  on  an  intermediate 
face  than  on  the  clinopinacoid  itself.  Since  the  extinction  angle 
on  crystal  faces  that  are  easily  determined  has  a  characteristic 
value  for  any  given  substance,  it  should  be  given  for  the  prism 
faces  as  well  as  for  the  clinopinacoid,  especially  in  the  case  of 
monoclinic  substances  having  a  good  cleavage  parallel  to  the 
prism,  for  if  the  prism  angle  is  known,  the  true  optic  angle  can  be 
calculated  from  these  values. 

Diopside  with  an  extinction  angle  of  38°  on  the  clinopinacoid  is  the  best 
example  for  illustrating  this  relationship.  The  crystal  is  mounted  on  a 
otation  apparatus,  as  described  in  the  appendix,  and  the  extinction  meas- 
ured on  the  faces  inclined  at  least 
10°  to  each  other  in  the  zone  of  the 
c  axis.  These  values  are  plotted 
as  ordinates,  while  the  abscissas 
represent  the  inclination  of  the 
various  faces  to  the  clinopinacoid, 
Fig.  93.  The  extinction  for  any 
face  lying  in  that  zone  can  be  read 
from  the  curve  plotted  from  this 
data.  Thus,  for  example,  a  prism 
is  inclined  about  45°  to  the  clino- 
pinacoid, the  prism  angle  being 
not  far  from  90°  in  pyroxene,  and 

the  extinction  on  that  face  is  about  30°.  Great  importance  must  be  at- 
tached to  the  character  of  such  curves  in  the  investigation  of  thin  sections. 
The  extinction  on  the  orthopinacoid  is  symmetrical,  but  the  angle  increases 
very  rapidly  on  faces  to  either  side  so  that  parallel  extinction  is  obtained 
only  on  faces  very  accurately  orientated,  as  is  rarely  the  case  in  thin  sec- 
tions. It  is  therefore  easy  to  reach  the  false  conclusion  that  the  mineral 
is  triclinic  because  it  shows  oblique  extinction  in  all  sections. 

In  triclinic  minerals  the  determination  of  the  extinction  angles 
is  of  value  only  when  the  orientation  of  the  face  upon  which 


38" 
30 

2<T 
10 

0° 
( 

0 

—  —  . 

—  ^ 



\ 

X. 

^\ 

N 

\ 

)     10      20     30     40     50     60     70     80    9( 
LO                                110                               10 

FIG.  93. — Extinction  Curve  for  Diopside. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  71 

they  are  observed  is  accurately  known.  The  orientation  of  the 
clinopinacoid  of  monoclinic  crystals  can  be  easily  determined 
by  convergent  light,  as  will  be  described  later,  because  this  face 
is  either  parallel  or  perpendicular  to  the  plane  of  the  optic  axes, 
but  such  determinations  are  generally  impossible  in  triclinic 
minerals.  However,  now  and  then  the  direction  of  cleavage 
cracks,  twinning,  etc..  offer  a  clue,  and  the  measurement  of  the 
extinction  angle  in  triclinic  minerals  is  important  only  in  such 
sections.  Extinction  is  of  especial  importance  in  the  investiga- 
tion of  cleavage  pieces. 

Stauroscopes. — The  simple  method  of  ascertaining  the  position  of  ab- 
solute darkness,  described  above,  is  not  sufficient  for  refined  measurements 
and  a  number  of  methods  have  been  devised  which  yield  more  accurate 
results,  especially  with  colorless  minerals.  After  the  position  of  extinction 
has  been  found  as  accurately  as  possible  with  crossed  nicols,  one  observes 
upon  rotating  the  analyzer,  whether  in  all  positions  of  the  nicols  it  shows 
the  same  tint  as  the  surrounding  field.  The  position  is  correct  only  when 
that  is  the  case. 

There  are  a  number  of  accessory  devices,  which  can  be  used  for  special 
purposes.  These  are  all  grouped  together  under  the  name  of  stauroscopes. 
The  most  important  are : 

1.  The  Kobell  stauroscope. 

2.  The  Brezina  double  plate. 

3.  The  Caldron  double  plate. 

4.  The  Bertrand  plate. 

5.  The  sensitive  tint  plate,  gypsum  test  plate. 

6.  The  Bravais  double  plate. 

7.  The  Biot  quartz  plate. 

8.  The  half  shadow  polarizer. 

9.  The  twin  polarizer. 

1.  The  Kobell  stauroscope  consists  of  a  plate  of  calcite  cut  perpendicular 
to  the  optic  axis,  and  can  be  inserted  between  the  upper  lens  of  the  ocular, 
and  the  ocular  analyzer  placed  over  it.     An  interference  figure  of  an  opti- 
cally uniaxial  crystal  is  seen,  but  it  is  destroyed  as  soon  as  even  the  slightest 
double  refracting  substance  is  placed  between  the  two  nicols.     The  vibration 
directions  of  the  crystal  studied  are  properly  placed  only  when  the  inter- 
ference figure  is  perfectly  restored. 

2.  The  Brezina  double  plate  consists  of  two  plates  of  calcite  placed  side 
by  side.     They  are  cut  equally  oblique  to  the  base,  but  in  opposite  directions 
and  are  symmetrical  with  respect  to  the  plane  of  contact  between  them. 
The  interference  figure  in  this  case  shows  a  single  band  surrounded  by 
colored  curves  in  the  middle  of  the  field  of  vision.     If  the  vibration  directions 
of  the  crystal  are  not  exactly  the  same  as  those  of  the  nicols  that  part  of  the 
band  over  the  crystal  appears  to  be  broken  off  from  the  rest  of  the  dark  bar. 

3.  The  Caldron  double  plate  is  placed  within  an  ocular  of  its  own,  because 
the  inserted  plate  demands  a  greater  distance  between  the  ocular  lenses  than 


72 


PETROGRAPHIC  METHODS 


is  usual.  It  consists  of  two  wedge-shaped  pieces  of  calcite  so  placed  that 
the  plane  of  contact  between  them,  when  viewed  from  above,  appears  as  a 
sharp  line  and  the  vibration  directions  in  the  two  halves  are  symmetrical  to 
it.  If  this  border  line  is  exactly  parallel  to  the  vibration  directions  of  the 
two  nicols,  each  half  shows  the  same  degree  of  extinction.  If  a  double  re- 
fracting plate  is  placed  in  the  path  of  the  rays,  and  its  vibration  directions 
do  not  coincide  with  those  of  the  nicols,  the  luminosity  of  the  two  halves 
of  the  double  plate  does  not  change  uniformly  because  their  vibration  direc- 
tions are  oblique  to  each  other.  Equal  luminosity  can  only  be  regained  by 
orientating  the  extinction  directions  of  the  crystal  plate  accurately. 

4.  The  Bertrand  plate,  like  the  Caldron,  is  usually  placed  in  an  ocular 
of  its  own.     It  consists  of  four  sectors  of  alternating  right  and  left  rotating 
quartz  cut  perpendicular  to  the  optic  axis  with  a  thickness  of  about  2 . 5  mm. 
The  interference  colors  of  the  four  sections  are  bluish-white,  but  they  are 
changed    uniformly    when    a    double    refracting 
crystal  is  placed  on  the  stage  with  its  vibration 
directions  not  exactly  parallel  to  those  of  the 
nicols,  Fig.   94.     If  they  are  parallel,   the  four 
sectors  must  all  have  and  retain  the  same  color 
when  the  nicols  are  rotated  horizontally. 

5.  The  sensitive  tint,  violet  I,  Fig.  96,  page 
75,  at  the  beginning  of  the  second  order  of 
colors  is  produced  by  a  thin  plate  of  a  colorless 
feebly  refracting  mineral,  quartz  or  gypsum,  of 
the  proper  thickness  to  give  this  particular 
interference  color  between  crossed  nicols.  It  is 

fastened  between  two  small  strips  of  glass  and  inserted  into  a  slot  above  the 
objective  in  such  a  manner  that  its  vibration  directions  are  at  45°  to  those  of 
the  nicols.  As  shown  in  Fig.  96  a  retardation  of  0 . 00001  mm.  is  sufficient 
to  change  this  color  to  purple  on  the  one  side  or  to  indigo  on  the  other.  The 
same  figure  also  shows  that  violet  I  is  more  susceptible  to  a  distinct  change 
of  color  than  all  the  interference  colors  occurring  between  crossed  nicols. 
Deep  violet  of  the  first  order,  which  is  produced  with  parallel  nicols  by  a  plate 
of  half  the  thickness  of  the  one  above  can  also  be  used  as  a  sensitive  tint. 
Violet  II,  at  the  beginning  of  the  third  order  of  colors,  although  it  is  often 
used  as  a  stauroscope,  is  however,  less  valuable  for  this  purpose  because 
with  it  marked  changes  of  color  take  place  only  with  great  differences  of 
thickness.  The  sensitive  tint  is  not  only  used  to  determine  the  vibration 
directions  accurately,  but  frequently  also  as  a  compensator  or  as  a  means 
of  recognizing  very  low  double  refraction,  because  the  insertion  of  a  crystal 
with  a  very  feeble  double  refraction  is  sufficient  to  influence  the  color 
decidedly. 

6.  A  Bravais  double  plate  extends  the  applicability  of  the  sensitive  tint. 
It  is  set  in  the  inner  focal  plane  of  the  ocular  and  on  account  of  its  thinness, 
the  ocular  scarcely  requires  correction.  One  of  the  small  plates  described 
under  5  is  cut  diagonal  to  its  vibration  directions  and  the  two  halves  are 
rotated  through  180°  to  each  other.  Thus  the  vibration  directions  of  the 
halves  are  at  90°  to  each  other,  Fig.  95.  The  plate  is  so  placed  in  an  ocular 
that  the  junction  of  the  two  parts  coincides  with  one  of  the  cross  hairs. 


FIG.  94. — Bertrand  Plate. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  73 

The  change  which  one  half  suffers  when  a  double  refracting  crystal  is  intro- 
duced is  reversed  in  the  other  half  and  these  two  lying  side  by  side  allow  the 
slightest  change  of  color  imaginable  to  be  recognized.  This  method  is  so 
accurate  that  the  double  refraction  of  a  glass  cube  can  be  observed  when 
pressed  between  the  fingers.  This  is  the  most  accurate  method  of  all  for 
recognizing  the  double  refraction  as  well  as  for  determining  the  extinction 
direction.  The  determination  of  the  latter  is,  however,  less  accurate 
because  it  depends  upon  the  change  of  the  interference  colors  and  cannot 
be  made  in  monochromatic  light,  i.  e.,  this 
method  cannot  be  used  for  determining  accur- 
ately the  extinction  directions  of  monoclinic 
and  triclinic  crystals. 

7.  The  Biot  quartz  plate,  cut  perpendicular 
to   the    c  axis,   is  3 . 75  mm.  thick  and   upon 
rotating  the  analyzer  gives  various  character- 
istic  interference    colors   between   two   nicols. 
In  a  definite  position  of  the  analyzer  the,  deli- 
cate   violet  of  the   sensitive  tint  is  obtained. 
This    serves    ostensibly  for  the  determination 

of  the  extinction  directions  of  colorless  crystals,  Fl0'  95.-Bravais  Double  Plate, 
while  other  tints  give  better  results  with  colored 

substances.  When  the  proper  position  of  extinction  has  been  found,  it 
can  be  checked  by  rotating  the  analyzer  and  noting  the  interference  colors. 
The  Biot  quartz  plate  is  generally  so  mounted  that  it  can  be  inserted  in 
the  slot  above  the  objective. 

8.  The  half  shadow  polarizer  is  a  nicol  which  has  been  cut  through  the 
middle  in  a  direction  slightly  oblique  to  its  principal  section  and  the  pieces 
recemented  together  in  reversed  positions.     The  vibration  directions  form 
a  small  angle  with  each  other.     It  is  used  in  place  of  the  polarizer  and  the 
two  halves  appear  in  equal  half  shadows  when  the  vibration  direction  of  the 
analyzer  is  parallel  to  the  line  of  division. 

9.  The  twin  polarizer  differs  from  the  above  in  that  the  vibration  directions 
in  the  two  halves  are  at  90°  to  each  other.     The  application  of  both  devices 
is  very  limited  because  the  line,  of  junction  and  the  crystal  must  be  seen 
sharply  at  the  same  time  which  means  that  only  objectives  with  large  focal 
lengths  can  be  used. 

Those  stauroscopes  which  depend  upon  a  change  of  interference  colors 
can  only  be  employed  for  determinations  in  white  light.  They  cannot 
be  used  when  accurate  determinations  of  the  extinction  directions  of  strongly 
dispersing  monoclinic  and  triclinic  crystals  are  to  be  made.  This  deter- 
mination must  be  carried  out  in  monochromatic  light.  A  Brezina  double 
plate  or  Kobell  stauroscope  is  to  be  preferred  in  such  cases. 

Strength  of  the  Double  Refraction,  Interference  Colors. — The 

two  rays  into  which  light  is  resolved  by  a  double  refracting 
crystal  are  propagated  differently  in  the  crystal.  Upon  emerging 
from  the  crystal  one  ray  is  retarded  a  certain  amount  behind  the 
other  and  this  retardation  depends  upon  the  difference  of  elas- 


74  PETROGRAPHIC  METHODS 

ticity  of  the  two  vibration  directions  perpendicular  to  the  direc- 
tion of  propagation,  and  upon  the  thickness  of  the  crystal. 
When  the  two  rays  are  brought  back  to  one  plane  of  vibration 
by  the  analyzer  they  show  a  difference  of  phase.  The  rule  is  that 
two  plane  polarized  rays  produced  by  the  double  refraction  of  a 
single  plane  polarized  ray  of  light,  interfere  with  each  other  when 
they  are  reverted  again  to  the  same  plane  of  polarization. 

Since  ordinary  light  is  composed  of  various  colors  with  different 
wave  lengths,  X,  the  phasal  difference  will  be  different  for  the 
various  colors  for  a  definite  thickness  of  the  crystal,  which  corre- 
sponds to  a  fixed  retardation  of  one  ray  over  the  other.  For  a 
definite  color  the  two  rays  are  compounded  by  the  interference 
and  that  color  appears  with  especial  brilliancy,  while  for  other 
colors  the  two  vibrations  destroy  each  other  and  no  light  of  that 
color  passes  through  the  analyzer.  Thus,  instead  of  white  light, 
colors  appear  and  they  are  called  interference  colors. 

There  are  two  cases  to  be  distinguished  in  the  interference 
of  light  depending  upon  whether  the  plane  of  vibration  of  the 
analyzer  is  parallel  or  perpendicular  to  that  of  the  polarizer.  In 
the  first  case  the  rays  emerging  from  the  crystal  interfere  with 
the  same  phase,  and  in  the  latter  case  with  reversed  phase.  With 
parallel  nicols  those  colors  appear  with  greatest  intensity  which 
are  extinguished  with  crossed  nicols,  and  vice  versa,  so  that  the 
interference  colors  observed  with  parallel  nicols  are  complemen- 
tary to  those  seen  between  crossed  nicols. 

Let  us  consider  more  in  detail  the  latter  case,  which  is  prac- 
tically the  only  one  used  in  microscopic  technic,  and  assume 
for  the  sake  of  simplicity,  that  with  a  double  refracting  crystal 
of  definite  thickness  the  same  absolute  retardation  of  the  two 
rays  is  produced  for  all  colors.  The  wave  length  A  of  extreme 
violet  amounts  to  about  380  ///*,  that  of  extreme  red  about  775 
fjLjji,  millionths  millimeters  or  microns.  With  crossed  nicols  those 
colors  are  emphasized  which  interfere  with  a  phasal  difference  of 
1/2  ^  or  an  odd  multiple  of  1/2  X,  while  those  colors  are  ex- 
tinguished which  interfere  with  a  retardation  of  one  or  several 
whole  wave  lengths.  The  interference  color  of  a  crystal  can  be 
determined  when  the  thickness  and  double  refraction  are  known, 
because  the  retardation  is  dependent  upon  both  of  these  factors. 
If  the  thickness  of  the  preparation  is  known,  the  double  refrac- 
tion can  be  determined  from  the  interference  color,  or  inversely 
if  the  double  refraction  is  known,  the  thickness  can  be  estimated. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  75 


+  Nicols 

II  Nicols 

white 

steel-gray               50 

white 

lavender-gray  __.100 
grayish-blue  150 
greenish-white    200 

^     yellowish-white 
O2-__brownish-white 

brownish-yellow 

-,,^_ 

white                   250 

FVA- 

light-red 

light-yellow.       .300 
bright-yellow—  .350 

orange-yellow.     430 

EK\ 

.carmine 

reddish-brown 
deep  violet 

7"P  /  \ 

cy2\- 

S^,                          indigo 

blue 

-H\ 
rt\ 

AHX- 

grayish-blue 

orange                  450 

bluish-green 
pale-green 

reddish-orange  ..500 
red                        530 

-F\ 

7?\ 

deep-red-          -550 

purple-                  565 

light-green 
83    greenish-yellow 
-  golden-yellow 

violet  I                 575 

indigo.                   .589 

-D\  
-CX 

—H%\- 

sky-blue               664 

®                  _orange 

greenish-blue      728 

-B\ 

0 
brownish-orange 

green                    747 

F%\- 

0        light-carmine 

green  800 

-AX 

-H2\ 

E%\- 

<-». 
_purple 

yellowish-green  850 
yellow                   910 

violet 

-G2X 

0 

indigo 

orange.  __.  948 

"t_           _  dark-blue 

orange-red          1000 

-F2\ 

C3/2X_ 

greenish-blue 

red                      1060 

-E2\ 

9 
green 

violet-red             1100 

A3/2X_ 
G%\ 

violet  II              1128 

y  ello  w  i  sh-green 

indigo  .        1151 

F%.\- 

.yellow 

iR2X 

Fio.  96.— The  Interference  Colors. 


76  PETROGRAPHIC  METHODS 

If  it  is  assumed  that  the  retardation  of  the  two  rays  in  a  crystal 
illuminated  by  polarized  light  amounts  to  only  50  ///*,  Fig.  96, 
a  small  phasal  difference  will  be  experienced  by  all  colors  upon 
emerging  from  the  crystal,  and  small  but  equal  amounts  of  light 
for  each  color  will  pass  through  the  analyzer,  and  the  impression 
of  a  gray  color  is  made  between  crossed  nicols,  +nicols.  If  the 
retardation  is  a  little  greater,  violet  will  be  more  prominent,  giving 
a  lavender  gray  tint.  The  other  colors  follow  in  order,  grayish- 
blue,  grayish-white,  white  and  yellow,  until  when  the  retardation 
is  about  300  /*/*,  or  about  half  the  wave  length  of  very  intense 
yellow,  that  color  is  especially  bright,  while  at  the  same  time 
extreme  violet,  /}  =  380,  is  almost  entirely  extinguished.  The 
general  impression  is  that  of  a  brilliant  yellow  color.  If  these 
interference  colors  are  observed  with  a  spectroscope,  the  yellow 
part  of  the  spectrum  will  appear  brilliantly  illuminated,  but 
diminishes  toward  both  sides  until  where  violet  is  reached  no 
light  is  visible. 

If  the  retardation  is  about  twice  as  great,  or  about  575  /*/*, 
it  equals  the  wave  length  of  yellow,  lying  near  the  Na  line.  This 
color  will  be  destroyed,  while  a  brilliant  violet  interferes  with 
nearly  1J  ^  phasal  difference,  and  appears  with  double  inten- 
sity, while  at  the  other  side  of  the  spectrum  some  is  visible. 
The  resulting  interference  color  is  a  reddish-violet,  violet  I,  Fig. 
96.  When  studied  spectroscopically  the  violet  part  of  the 
spectrum  is  seen  to  be  strongly  illuminated  and  a  dark  line  ap- 
pears in  the  yellow.  Violet  is  the  least  luminous  color  in  the 
spectrum  and  is  therefore  greatly  influenced  by  extremely  small 
changes  in  the  retardation  as  shown  in  Fig.  96.  It  is  called  the 
sensitive  red  or  violet. 

In  the  same  manner  the  changes  which  white  light  undergoes 
with  different  retardations  can  be  calculated,  and  it  is  found 
that  blue  follows  violet,  green  follows  blue,  etc.,  until  finally 
violet  is  reached  again,  and  the  same  order  of  colors  repeated. 
A  series  of  colors  from  one  violet  to  another  is  called  an 
order  and  distinction  is  made  between  first,  second,  third 
orders,  etc. 

Let  us  consider  more  carefully  a  color  of  the  third  order,  one 
which  results  by  a  retardation  of  1500  /*//.  This  is  twice  the 
wave  length  of  brilliant  red,  three  times  that  of  a  blue-green, 
and  four  times  that  of  extreme  violet.  These  colors  will  all  be 
extinguished,  while  yellow  and  blue  will  be  intensified.  The 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  77 

total  result  is  a  yellowish  tint,  because  of  the  greater  intensity 
of  the  yellow  part  of  the  spectrum.  This  color,  however,  appears 
somewhat  mixed  when  compared  with  the  one  above,  and  does 
not  give  the  impression  of  a  pure  yellow.  If  the  retardation 
amounts  to  about  5000  /*/<,  it  corresponds  to  7  A  for  intense  red, 
8  X  for  orange,  9  X  for  yellowish-green,  10  X  for  green,  11  X 
for  blue,  12  X  for  bluish-violet,  and  13  X  for  violet.  These  colors 
will  all  be  extinguished,  while  the  intermediate  red,  yellow,  green, 
blue,  and  violet  colors  are  intensified;  when  these  colors  are  united 
they  form  white.  This  interference  color  is  called  white  of  the 
higher  order  in  contradistinction  to  the  white  of  the  first  order 
produced  by  very  slight  retardations.  It  may  be  very  difficult 
for  beginners  to  distinguish  between  these  two  white  interfer- 
ence colors.  The  white  of  the  higher  order  produced  by  mani- 
fold interference  frequently  has  a  delicate  appearance  similar  to 
mother  of  pearl,  while  white  of  the  first  order  is  generally  mixed 
with  gray,  green,  or  yellow. 

The  differentiation  of  the  various  orders  of  interference  colors 
is  quite  difficult  for  a  beginner.  In  a  normal  case  the  colors  of 
the  first  order  with  their  deep  hard  tones  can  be  separated  from 
those  of  the  second  order,  which  are  unusually  brilliant,  luminous 
colors.  In  the  third  order  the  colors  become  duller  and  more 
mixed,  and  farther  up  the  brilliancy  is  rapidly  lost,  until  in 
the  fifth  and  sixth  orders  white  of  the  higher  order  is  reached. 
It  may  be  held  as  a  rule  for  the  estimation  of  interference  colors 
under  all  conditions  that  the  colors  appear  harder  and  purer  the 
lower  they  are.  This  difference  will  appear  quite  distinctly 
even  to  an  inexperienced  eye  by  comparing  the  red  of  the  first 
and  second  orders. 

The  two  nicols  are  generally  crossed  for  observing  the  inter- 
ference colors  and  only  in  comparatively  rare  cases  is  observation 
between  parallel  nicols  advisable.  Such  a  case  would  be  that  of  a 
very  low  double  refracting  crystal,  which  is  not  very  bright  be- 
tween crossed  nicols,  and  the  presence  of  double  refraction  can- 
not be  positively  determined.  Fairly  distinct  colors  may  be 
observed  then  between  parallel  nicols,  see  Fig.  96. 

Modification  of  the  Interference  Colors. — In  deriving  the  interference 
colors  above,  it  was  assumed  that  the  retardation  of  one  ray  over  another 
was  the  same  for  all  colors,  but  this  is  by  no  means  the  case.  Just  as  the 
index  of  refraction  is  different  for  different  colors,  so  the  difference  between 
the  indices,  which  produces  the  interference  color  in  polarized  light,  varies. 


78 


PETROGRAPHIC  METHODS 


This  can  be  deduced  from  the  table  of  the  indices  of  calcite  for  various  lines 
of  the  spectrum. 


A 1.650 

B 1.653 

D 1.658 

F..  .  1.668 


H.. 


1.683 


1.483 
1.484 
1.486 
1.491 
1.498 


r-OL 
0.167 
0.169 
0.172 
0.177 
0.185 


The  interference  colors  for  various  substances  are  not  entirely  the  same, 
although  the  mean  retardation  may  be  identical,  because  of  the  differences 
in  indices  of  refraction  for  the  various  colors.  These  differences  are  generally 
not  very  large  but  in  spite  of  that,  the  difficulty  of  observing  the  inter- 
ference colors  is  considerably  increased  by  the  dispersion  of  the  double 
refraction  even  in  ordinary  cases.  If  that  is  the  case,  as  in  the  example 
already  cited,  calcite,  and  likewise  in  quartz  and  most  of  the  other  rock- 
forming  minerals,  the  double  refraction  for  red  is  less  than  that  for  violet 
and  there  appears  according  to  Becke  a  brilliant  supernormal  interference 
color  of  the  first  order.  With  greater  dispersion  it  may  attain  the  luminous 
properties  of  the  second  order.  If  7- — a  is  greater  for  red  than  for  violet, 
which  is  more  rarely  the  case,  the  interference  colors  are  duller,  subnormal. 
Naturally  this  behavior  often  causes  great  inconvenience  in  the  methods  of 
compensation  to  be  described  later.  The  various  colors  cannot  be  entirely 
compensated  because  they  have  varying  double  refraction  in  various  parts 
of  the  spectrum. 

It  is  possible  that  in  the  dispersion,  the  double  refraction  for  some  color 
may  be  zero,  so  that,  for  one  end  of  the  spectrum  the  double  refraction 
will  be  positive  and  for  tne  other  end  negative,  while  for  some  intermediate 
color  it  will  be  zero.  The  latter  color  will  be  extinguished  and  instead  of  a 


ABC 


7-CT  +  0.001  ±0.0  -0.002 

FIG.  97. — Double  Refraction  of  Fuggerite  for  Different  Parts  of  the  Spectrum. 


normal  gray  or  grayish-blue  color  of  the  lowest  order,  a  deep  color  appears 
that  is  complementary  to  the  extinguished  color.  Such  interference  colors 
are  called  abnormal  in  the  narrower  sense. 

This  is  the  case  in  the  mineral  fuggerite  with  low  double  refraction,  Fig. 
97.  It  is  weakly  positive  for  one  end  of  the  spectrum  and  weakly  negative 
for  the  other,  but  between  the  two,  near  the  Na  line,  the  double  refraction 
is  0.  That  color  is  always  extinguished  in  polarized  light  by  a  plate  of  any 
thickness,  while  the  light  emerging  from  the  preparation  combines  to  form 
not  a  gray  of  the  first  order  characteristic  of  low  double  refraction,  but  a 
color  complementary  to  the  yellow,  that  is,  a  deep  blue  which  becomes  more 
luminous  the  thicker  the  slide. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  79 

The  blue  color  can  be  seen  only  after  prolonged  observation  in  very  thin 
slides,  but  in  thicker  slides  it  may  appear  as  a  very  brilliant  Prussian  blue. 
An  experienced  eye  can  recognize  at  once  the  difference  in  shade  between  it 
and  a  normal  interference  color.  A  beginner  can  easily  recognize  the  ab- 
normal property  of  the  color  by  making  use  of  the  \  \  mica  plate  as  a  com- 
pensator. Anomalous  interference  colors  are  never  entirely  compensated 
by  the  plate,  but  are  changed  to  a  dirty  brownish-gray  when  the  vibration 
directions  of  the  mineral  are  crossed  with  respect  to  those  of  the  com- 
pensator. Under  similar  conditions  indigo  or  blue  of  the  second  order, 
which  may  appear  similar  to  the  abnormal  color,  are  reduced  to  a  brilliant 
orange  by  a  reduction  of  150  /*/*,  Fig.  91,  so  that  the  anomalous  property 
of  the  other  color  can  be  seen  most  distinctly.  The  same  is  true  if  the 
equivalent  directions  are  parallel  in  the  two  objects.  The  anomalous  blue 
changes  to  a  dirty  white,  while  the  normal  passes  over  into  a  brilliant 
greenish-blue.  Various  complementary  colors  may  occur  as  abnormal 
interference  colors  according  to  the  location  in  the  spectrum  of  the  zero 
point  of  double  refraction. 

Another  series  of  abnormal  interference  colors  depends  upon  the  dis- 
persion of  the  vibration  directions  in  monoclinic  and  triclintc  crystals.  In 
cross  sections  of  these  minerals  the  vibration  directions  for  different  colors 
differ  from  each  other.  In  consequence  of  this  all  colors  are  not  extinguished 
at  the  same  time  upon  rotating  between  crossed  nicols.  Instead  of  complete 
extinction  there  is  a  change  of  colors  from  yellow  to  violet.  The  properties 
of  the  interference  colors  are  materially  influenced  by  this  dispersion  when 
the  vibration  directions  are  at  45°  to  those  of  the  nicols.  In  sections  nearly 
perpendicular  to  an  optic  axis  extremely  abnormal  colors  may  occur. 
Such  interference  colors  are  called  dispersion  colors.  It  must  also  be 
remembered  that  the  interference  colors  are  greatly  modified  by  the  color 
of  the  mineral  itself  and  by  considerable  differences  of  absorption  in  different 
directions. 

The  larger  the  angle  of  aperture  of  the  illuminating  and  observing  systems 
of  the  microscope  the  less  pure  will  the  interference  colors  appear.  This  is 
partly  because  light  passing  through  the  lenses  in  a  very  oblique  direction 
is  partially  repolarized,  and  partly  because  light  travels  through  the  crystal 
in  very  diverging  directions  and  therefore  suffers  different  retardations. 
The  polarization  colors  are  mixed  with  each  other  and  this  naturally  makes 
exact  determinations  very  difficult.  When  wide-angled  systems  are  used, 
the  illuminating  cone  of  light  should  be  narrowed  down  for  observations  in 
parallel  polarized  light  as  far  as  the  source  of  light  will  permit. 

Measurement  of  Double  Refraction  by  Means  of  Interference 
Colors. — An  interference  color  depends  upon  the  difference  of 
retardation  of  one  ray  over  another.  In  one  and  the  same 
substance  it  bears  direct  relation  to  the  thickness  of  the  plate. 
Therefore,  the  color  can  be  used  to  determine  the  double  refrac- 
tion or  the  thickness  of  a  slide.  The  appended  table  shows  the 
thickness  in  millimeters  which  plates  of  various  crystals,  cut 
parallel  to  the  optic  axis  or  to  the  axial  plane,  must  have  in 


80  PETROGRAPHIC  METHODS 

order  to  give  a  brilliant   reddish-violet  of  the  first  order,  the 
retardation  being  0.000575  mm. 

y-a.       Thickness  in  mm. 

Mercurous  chloride 0 . 640  0 . 00086 

Triphenyl  benzol 0 . 348  0 . 0016 

Rutile  and  sulphur 0 . 287  0 . 0019 

Calcite 0.172  0.0032 

Olivine 0.036  0.015 

Quartz 0.009  0.06 

Apatite 0 . 004  0.14 

Apophyllite 0 . 001  0 . 55 

A  few  of  the  more  important  rock-forming  minerals  show 
the  following  maximum  interference  colors  in  a  normal  section 
about  0.03  mm.  thick: 

a.  Gray    to    grayish- white;    pennine,     vesuvianite,    apatite, 
zoisite,  nepheline. 

b.  White    to    yellowish- white ;    feldspar,    quartz,    serpentine, 
topaz,  cordierite,  enstatite,  andalusite,  clinochlore,  gypsum. 

c.  Yellow  to  orange;  hypersthene,  wollastonite,  cyanite. 

d.  Red  I  to  indigo;  tourmaline,  sillimanite,  glaucophane. 

e.  Blue  to  green;  augite,  hornblende. 

/.  Yellow  to  orange;  chondrodite,  olivine. 

g.  Red  II  to  blue;  mica,  segirine,  etc. 

If  the  section  is  0 . 04  mm.  thick  most  of  the  minerals  under  b 
above  will  be  yellow  to  orange  and  the  interference  colors  of  the 
others  will  be  raised  accordingly,  mica,  for  example,  assuming 
a  dull  color  of  the  fourth  order.  If  on  the  other  hand  the  thick- 
ness of  the  slide  is  only  0.02  mm.  all  the  minerals  under  b  will 
have  a  dull  gray  interference  color  of  the  lowest  order,  and  the 
color  for  mica  will  scarcely  be  as  high  as  the  middle  of  the 
second  order.  In  either  case  the  interference  color  is  of  no  use 
in  determining  the  mineral. 

Various  methods  can  be  used  to  determine  the  double  refrac- 
tion of  a  crystal  by  means  of  the  interference  colors  observed 
between  crossed  nicols.  These  methods  may  depend  in  part 
upon  a  comparison  of  the  interference  color  of  an  unknown, 
with  that  of  a  known  crystal,  and  in  part  upon  the  compensation 
of  the  double  refraction  by  the  use  of  the  compensators  to  be 
described  later.  Each  of  the  latter  shows  a  series  of  interfer- 
ence colors  so  placed  that  they  can  be  compared  directly.  It  is 
necessary  to  know  accurately  the  thickness  of  the  slide  and  the 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  SI 

orientation  of  the  cross  section  for  all  methods  of  measuring 
the  double  refraction  by  means  of  the  interference  colors. 

Approximate  determinations  are  the  simplest  and  give  results 
most  rapidly.  For  this  purpose  the  interference  color  of  the 
object  is  compared  with  a  table  of  interference  colors  such  as  the 
one  devised  by  Michel-Levy  and  Lacroix.  After  a  little  practice 
very  good  results  may  be  obtained  with  it. 

Somewhat  more  accurate  determinations  can  be  made  with  the  quartz 
wedge  comparator,  likewise  constructed  by  Michel-LeVy.  A  cross  section 
of  it  is  shown  in  Fig.  98.  It  is  attached  to  the  side  of  an  ocular  O.  The 
light  L  is  reflected  into  it  by  the  mirror  S,  and  after  total  reflection  by  the 
prism  C,  passes  through  the  quartz  wedge  G  between  two  nicols  A  and  P. 
The  quartz  wedge  can  be  moved  by  means  of  a  screw.  The  light  finally 
passes  into  a  prism  R,  the  reflecting  face  of  which  is  silvered,  and  is  totally 
reflected  into  the  field  of  vision  of  the  ocular.  There  is  a  small,  round, 


\ 
L 

FIG.  98. — Quartz  Wedge  Comparator  devised  by  Michel-Levy. 

unsilvered  spot  in  the  center  of  the  latter  prism  and  upon  this,  the  small 
prism  R'  is  cemented.  Through  this  opening  the  interference  color  of  the 
crystal  M  lying  under  the  objective  is  observed  while  the  rest  of  the  field, 
which  receives  its  light  from  the  totally  reflecting  prisms,  shows  the  inter- 
ference color  of  that  portion  of  the  quartz  wedge  which  is  in  the  path  ,of  the 
rays.  A  circular  area  is  observed  in  the  field  of  the  ocular,  which  shows 
the  interference  color  of  the  mineral  lying  upon  the  stage  and  this  is  sur- 
rounded by  a  ring  of  the  interference  color  from  the  quartz  wedge.  The 
latter  can  be  made  to  agree  with  the  color  of  the  inner  area  by  moving  the 
wedge.  The  thickness  of  the  quartz  wedge  at  this  particular  point  can  be 
observed  on  a  scale  and  the  order  of  the  interference  color  determined 
directly. 

This  apparatus  is  very  interesting,  but  is  only  applicable  in  exceptional 
cases.  The  modifications  which  the  interference  colors  undergo,  (1)  by  the 
varying  double  refraction  for  different  colors,  (2)  by  the  different  indices  of 
refraction  of  the  crystal,  and  (3)  by  its  transparency  are  sufficient  to  make 


82    ,  PETROGRAPHIC  METHODS 

an  absolute  comparison  of  the  interference  colors  very  difficult.  In  addition 
there  are  a  number  of  obstacles  in  the  determination  of  the  interference 
colors  of  which  the  difference  in  the  illumination  of  the  objects  to  be 
compared  may  be  mentioned,  so  that  this  method  can  be  used  advantage- 
ously with  colorless  minerals  only.  The  thickness  of  the  slide  can  be 
measured  as  it  is  being  ground,  by  placing  small  basal  cleavage  pieces  of 
barite  on  each  of  the  four  corners  of  the  slide  and  grinding  them  down 
with  it.  The  interference  color  of  the  barite  can  be  used  to  indicate  the 
thickness,  provided  the  surface  is  not  clouded  as  is  very  often  the  case. 

This  determination  of  thickness  is  very  accurate  when  the  interference 
colors  are  gray  or  grayish-blue  of  the  lowest  order.  It  has  been  proposed  in 
such  cases  to  place  a  horizontal  mirror  directly  under  the  preparation. 
The  light  would  then  be  returned  through  the  analyzer  by  means  of  a  mirror 
in  the  tube  above  it  and  through  the  slide  to  the  mirror  below,  from  which 
it  would  be  reflected  a  second  time  through  the  slide  and  analyzer,  before 
it  reaches  the  eye,  and  the  impression  would  be  that  a  plate  of  double  the 
thickness  was  being  observed  between  parallel  nicols.  Even  in  such  cases 
more  reliable  results  are  obtained  by  means  of  compensators.  It  is  to 
be  noted  that  most  of  the  minerals  show  low  gray  colors  in  sections  with  an 
approximate  thickness  of  0.02  mm.  and  even  more  so  with  a  thickness  of 
0.01  mm.  It  is  advisable  for  ordinary  investigations  not  to  have  the 
slide  ground  too  thin. 

The  determination  of  the  double  refraction  in  thin  sections  by  interference 
colors  is  not  very  accurate,  because  of  the  fact  that  the  section  is  accurately 
orientated  only  in  exceptional  cases.  To  determine  the  difference  in 
velocity  of  the  two  rays  in  an  optically  uniaxial  mineral,  it  is  necessary  to 
use  a  section  cut  absolutely  parallel  to  the  optic  axis  because  the  true  value 
for  the  extraordinary  ray  can  be  obtained  only  in  such  sections,  and  in  like 
manner  the  true  value  of  ?  —  a  in  biaxial  substances  is  seen  only  in  a  section 
parallel  to  the  plane  of  the  optic  axes.  In  every  case  in  which  an  accurate 
determination  is  to  be  made,  the  orientation  of  the  cross  section  must  be 
accurately  established. 

Interference  Colors  in  Variously  Orientated  Cross  Sections.— 

Sections  parallel  to  the  optic  axis  show  the  highest  interference 
colors  produced  by  a  uniaxial  mineral.  The  colors  become 
lower  the  greater  the  inclination  of  the  section  to  the  optic  axis, 
until  with  an  inclination  of  90°  there  is  no  effect  on  parallel 
polarized  light  whatever.  Sections  of  optically  uniaxial  minerals 
cut  perpendicular  to  the  optic  axis  behave  in  perfectly,  parallel 
polarized  light  as  isotropic  substances.  However,  more  or  less 
convergent  rays  pass  through  microscopic  preparations,  and 
if  the  double  refraction  is  high,  or  the  thickness  of  the  slide  con- 
siderable, these  rays  passing  obliquely  through  the  sections 
produce  considerable  illumination  of  the  image,  because  they 
do  not  traverse  the  crystal  in  the  direction  of  its  isotropic  axis. 
Hence,  sections  of  uniaxial  minerals  perpendicular  to  the  optic 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  S3 

axis  are  weakly  illuminated  and  remain  so  upon  the  rotation 
through  360°.  This  corresponds  entirely  to  the  phenomenon, 
which  biaxial  crystals  cut  perpendicular  to  an  optic  axis  show, 
except,  as  will  be  pointed  out  directly,  the  cause  of  the  illumi- 
nation in  the  latter  case  is  for  the  most  part  different. 

Sections  of  biaxial  crystals  parallel  to  the  plane  of  the  optic 
axes  show  the  highest  interference  colors  of  all,  because  here  the 
maximum  and  minimum  velocities  enter  into  consideration. 
In  a  section  perpendicular  to  the  acute  bisectrix,  the  inter- 
ference colors  are  lower  the  smaller  the  angle  between  the  optic 
axes.  In  any  case  the  interference  color  of  such  a  section  is 
lower  than  that  of  a  section  perpendicular  to  the  obtuse  bisec- 
trix, the  interference  colors  of  which  more  nearly  approach  those 
of  a  section  parallel  to  the  axial  plane  the  larger  the  obtuse 
optic  angle.  In  a  section  perpendicular  to  an  optic  axis,  a  ray 
of  light  is  resolved  into  an  infinite  number  of  rays  lying  upon  the 
surface  of  a  cone  each  polarized  in  a  different  direction.  This 
phenomenon  is  called  interior  conical  refraction  and  the  result  is 
no  interference  color,  but  equal  illumination.  Such  sections  of 
biaxial  crystals  appear  light  between  crossed  nicols  and  remain 
uniformly  light  upon  a  complete  rotation  of  the  slide  through 
360°.  This  phenomenon  is  less  distinct  the  thinner  the  slide 
and  the  lower  the  double  refraction  of  the  crystal.  It  can 
scarcely  be  recognized  at  all  in  thin  sections  of  very  feebly 
double  refracting  crystals,  which  appear  uniformly  dark  in 
parallel  polarized  light. 

Chromoscope  for  Interference  Colors. — The  accurate  determi- 
nation of  the  interference  colors  is  one  of  the  most  important  pro- 
cesses in  the  technic  of  a  polarizing  microscope.  Even  the 
beginner  must  be  able  to  do  it  in  as  positive  a  manner  as  possible. 
The  differentiation  of  the  various  orders  of  color  is  quite  difficult, 
particularly  in  the  first  three  orders,  which  occur  most  frequently 
and  this  difficulty  can  be  overcome  only  by  careful  practice  and 
the  repeated  observation  of  interference  colors,  which  can  be 
easily  determined.  Various  methods  have  been  used  to  present 
the  subject  of  interference  colors  in  as  objective  a  manner  as 
possible.  At  first  a  color  table  was  prepared,  which  corresponded 
more  or  less  with  the  natural  colors  and  showed  an  arrangement 
of  the  important  series  of  colors  from  the  first  to  the  fourth 
orders.  Even  in  the  most  favorable  cases  a  poor  idea  of  the 
actual  conditions  could  be  obtained  from  it,  because  subjective 


84  PETROGRAPHIC  METHODS 

and  technical  factors  played  a  great  r61e  in  the  preparation  of 
the  colors,  which  made  an  approximation  to  the  true  properties 
of  the  original  very  difficult.  The  ink  used  generally  constitutes 
another  bad  factor,  for  aniline  dyes  are  always  used  for  this 
purpose,  but  they  fade  in  a  comparatively  short  time,  and  thus 
make  the  general  impression  even  more  inaccurate.  The  excel- 
lent reproductions  of  interference  colors  in  the  table  of  bire- 
fringences by  Michel-Levy  and  Lacroix  lose  their  true  value  for 
instructional  purposes  in  a  comparatively  short  time. 

The  interference  colors  are  a  simple  function  of  the  thickness 
of  a  crystal,  the  double  refraction  being  constant.  Therefore, 
a  thin  wedge  of  a  homogeneous  crystalline  substance,  observed  in 
white  parallel  polarized  light,  shows  the  colors  in  parallel  bands 
beside  each  other  analogous  to  a  table  of  colors.  Such  a  wedge 
usually  made  of  quartz  generally  accompanies  a  polarizing  micro- 
scope as  a  compensator.  With  a  length  of  4-5  cm.,  it  shows  the 
first  four  orders  of  colors  which  are  the  principal  ones  to  be 
distinguished.  Above  these  the  interference  colors  so  nearly 
approach  white  of  the  higher  order  that  they  can  scarcely  be  dis- 
tinguished even  by  a  skilled  observer.  If  such  a  wedge  is 
pushed  slowly  across  the  field  in  such  a  position  that  its  vibra- 
tion directions  are  at  45°  to  those  of  the  nicols,  the  various 
orders  of  color  can  be  observed  in  succession.  Only  a  small 
portion  of  the  series  can  be  seen  at  one  time  because  of  the 
narrowness  of  the  field  of  vision  of  the  microscope,  and  hence, 
analogous  colors  of  the  various  orders  cannot  be  observed 
simultaneously  so  as  to  be  compared  with  each  other,  the 
differentiation  of  which  is  of  great  importance.  Smaller  wedges 
of  this  character  could  be  made,  which,  when  observed  with  a 
low  power  objective  would  show  the  first  four  orders  of  color  in 
the  field  at  once.  Instead  of  such  a  wedge,  a  wedge-shaped 
crystal  of  ammonium  metavanadinate,  which  is  quit  easily 
prepared,  could  be  used  to  bring  the  interference  colors  beside 
each  other  in  distinct  bands  in  the  field  of  vision.  Careful  study 
of  a  single  color  is  interfered  with  by  the  narrowness  of  the  bands 
in  the  field,  and  also  by  the  fact  that  analogous  colors  of  the 
various  orders  are  separated  by  bands  of  different  colors. 

An  apparatus  devised  by  Weinschenk,  called  a  chromoscope  for 
interference  colors,  has  been  found  to  be  very  useful  in  these 
fundamental  studies.  It  consists  of  a  glass  plate  a,  Fig.  99;  as 
a  polarizer  and  an  arm  b  attached  at  the  angle  of  polarization  of 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  85 

the  glass  and  carrying  a  nicol  prism  d  that  can  be  rotated  and  used 
as  an  analyzer.  The  arm  also  bears  a  lens  e  with  a  two  fold  mag- 
nification and  so  arranged  that  it  can  be  thrown  out  of  the  line 
of  vision.  The  holder  c  is  so  placed  that  its  front  surface  corre- 
sponds to  the  normal  to  the  direction  of  polarization.  A  quartz 
wedge  8  cm.  long  and  3  cm.  wide  is  placed  in  the  holder  with  its 


FIG.  99 — Chromoscope  for  Interference  Colors. 

vibration  directions  at  45°  to  the  edges  of  the  holder.  Then  a 
mica  plate  of  the  same  size  is  placed  under  it  with  its  equivalent 
directions  crossed  so  that  with  it,  demonstrations  can  be  made 
with  the  lowest  interference  colors.  With  this  arrangement,  the 
first  four  orders  of  colors  from  low  gray  to  red  of  the  fourth  order 
are  obtained  in  regular,  broad,  parallel  bands.  By  inserting  the 
lens  the  magnification  is  doubled,  but  two  orders  of  colors  can 
still  be  seen  simultaneously.  Two  plane  parallel  gypsum  plates, 


FIG.  100.— Quartz  Wedge  in  Chromoscope  with  Parallel  Orientated  Gypsum  Plates  Red  I 

and  Red  II. 

each  8  cm.  long  but  only  1  cm.  wide,  with  vibration  directions  at 
45°  to  the  edges,  accompany  the  apparatus.  These  plates  alone 
yield  red  of  the  first  and  second  orders.  The  two  gypsum 
plates  are  then  placed  beside  each  other  on  the  quartz  wedge  so 
that  their  equivalent  directions  are  parallel,  the  lower  plate  show- 


86 


PETROGRAPHIC  METHODS 


ing  red  of  the  second  order,  the  upper  red  of  the  first  order,  as 
shown  in  Fig.  100.  From  the  top  downward,  bands  of  red  of  the 
first,  second  and  third  orders,  each  1  cm.  wide,  are  in  contact 
with  each  other  and  can  be  compared  directly.  Thus  the  phe- 
nomenon of  the  addition  of  double  refraction  can  be  studied. 
Fig.  101  shows  the  gypsum  plates  rotated  through  180°  from 
which  the  compensation  of  the  double  refraction  can  be  derived. 
The  numerals  in  Figs.  100  and  101  indicate  the  order  of  the  colors 
in  their  respective  positions.  The  longest  arrow  shows  the  di- 
rection of  the  fastest  ray  in  the  quartz  wedge,  the  smaller  ones 
being  used  for  the  gypsum  plates. 


FIG.  101. — Quartz  Wedge  in  Chromoscope  with  Oppositely  Orientated  Gypsum  Plates 

Red  I  and  Red  II. 

Interference  colors  that  occur  in  wedge-shaped  layers  within  a  slide  not 
infrequently  serve  to  determine  the  order  of  the  color  in  microscopic  study 
of  thin  sections  and  isolated  crystals.  They  often  aid  materially  in  denning 
the  crystal  form.  Wedge-shaped  individuals  may  be  formed  in  thin  section 
(1)  by  the  flaking  off  of  a  mineral  with  good  cleavage,  (2)  by  the  edge  of  the 
individual  being  quite  oblique  to  the  surface  of  the  slide,  and  (3)  by  the  edge 
of  the  slide  itself  being  somewhat  thinner,  which  is  generally  the  case.  The 
interference  color  in  the  principal  part  of  the  section  can  be  determined  by 
beginning  in  the  thinnest  place  where  the  color  is  lowest  and  counting  the 
bands  in  order,  i.e.,  noting  how  often  red  is  repeated.  In  the  case  of  oblique 
twinning  lamellae  of  very  strong  double  refracting  minerals,  such  color  bands 
produced  by  wedge-shaped  sections  often  lead  to  the  erroneous  conclusion 
that  there  is  a  whole  series  of  such  lamellae,  when  in  fact  there  is  but  a  single 
one  cut  across  very  obliquely.  An  example  of  this  phenomenon  is  shown 
in  Fig.  102,  which  represents  an  augite  crystal  cut  very  obliquely  through  a 
simple  twin. 

Interference  colors  on  the  wedge-shaped  outer  portions  of  isolated  crystals 
are  frequently  of  importance  in  determining  the  crystal  form,  as  for  ex- 
ample, the  bands  around  the  edge  of  an  artificial  crystal  of  anglesite,  Fig. 
103,  from  which  the  form  can  be  easily  observed.  They  also  aid  in  the 
approximate  determination  of  the  double  refraction.  If  these  bands  are 
very  close  to  the  edges  of  a  crystal  that  are  quite  flat,  it  is  evident  that  the 
substance  has  high  double  refraction,  etc. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  87 

Crystals  are  found  now  and  then  which  do  not  extinguish  at  all  when 
rotated  between  crossed  nicols,  but  show  a  change  in  interference  colors. 
In  a  very  few  cases  this  may  depend  upon  a  strong  dispersion  of  the  vibra- 
tion directions  or  upon  the  fact  that  the  crystal  is  twinned  and  the  twinning 
plane  is  approximately  parallel  to  the  plane  of  the  stage  of  the  microscope 
in  which  case  the  vibration  directions  of  the  two  individuals  will  be  oblique 
to  each  other.  When  the  vibration  directions  of  one  individual  are  parallel 
to  those  of  the  nicols,  the  interference  color  of  the  other  is  seen  and  vice 
versa,  while  in  intermediate  positions  transition  colors  are  present.  These 
two  cases  may  be  distinguished  by  observation  in  monochromatic  light 
when  there  will  be  complete  extinction  if  the  cause  of  the  change  of  color 
is  based  upon  dispersion  of  the  vibration  directions,  but  there  will  be  no 
extinction  in  the  case  of  overlapping  twins. 


FIG.  102. — Simple  Twinning  Lamellae        FIG.  103. — Artificial  Crystal  of  Lead  Sulphate 
in  Augite.  between  Crossed  Nicols. 

Character  of  the  Double  Refraction. — The  interference  color 
is  dependent  upon  the  retardation  of  one  ray  over  the  other  in  its 
passage  through  a  double  refracting  crystal.  In  a  plate  cut  par- 
allel to  the  optic  axis  of  a  negative  uniaxial  mineral  the  extra- 
ordinary ray,  vibrating  parallel  to  the  optic  axis,  exceeds  by  a 
definite  amount  the  ordinary,  vibrating  perpendicular  to  it  and 
is  refracted  less.  If  a  second  crystal  plate  is  laid  upon  the  first 
in  parallel  position,  that  ray,  which  in  the  first  traveled  with 
the  greater  velocity  and  less  refraction,  will  likewise  be  the  faster 
and  less  refracted  ray  in  the  second,  while  the  other  is  propagated 
in  each  case  with  a  smaller  velocity  and  greater  refraction,  Fig. 
104.  The  phasal  difference  between  the  rays  upon  emerging 
from  the  second  crystal  will  be  equal,  to  the  sum  of  the  retarda- 
tions in  each.  The  double  refraction  and  with  it  the  interference 


88 


PETROGRAPHIC  METHODS 


color  of  two  crystals  are  added  together  when  equivalent  vibra- 
tion directions  lie  parallel.  If  one  of  the  crystals  is  rotated  90° 
or  if,  as  represented  in  Fig.  105;  an  equivalent  plate  of  a  pos- 
itive crystal  is  used  instead  of  a  second  negative  one,  that  ray 
which  travels  in  the  first  with  the  greater  velocity  and  smaller 
refraction  will  be  propagated  in  the  second  with  a  smaller  velocity 
and  greater  refraction,  while  the  reverse  holds  for  the  other  ray. 
The  phasal  difference,  which  the  two  rays  show  upon  emergence 
from  this  combination,  will  be  equal  to  the  difference  of  the  re- 


FIG.  104. — Addition  FIG.  105. — Compensation 

of  the  Double  Refraction. 


tardations  in  each.  The  double  refraction  and  with  it  the  inter- 
ference color  of  two  crystals  is  compensated  when  equivalent 
vibration  directions  are  crossed. 

If  the  retardation  in  one  plate  is  equal  to  the  retardation  in 
the  other  a  combination  of  them  in  crossed  position  will  act 
as  an  isotropic  body  in  parallel  polarized  light,  i.e.,  the  double 
refraction  of  one  entirely  nullifies  the  double  refraction  of  the 
other,  as  respresented  in  Fig.  105. 

Compensators. — The  use  of  compensators  depends  upon  the 
phenomena  described  above.  They  serve  to  determine  the  char- 
acter of  the  double  refraction  by  raising  or  lowering  the  inter- 
ference color  and  sometimes  an  accurate  determination  of  the 
interference  color  can  be  made  by  perfect  compensation.  The 
most  important  compensators  are: 

1.  Quarter  undulation  mica  plate. 

2.  Plates  with  reddish-violet  I,  II,  etc. 

3.  Fedorow  mica  wedge. 

4.  Quartz  wedge. 

5.  Birefractometer. 

6.  Babinet  compensator. 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  89 

7.  Twin  compensator. 

8.  Biot  rotating  quartz. 

9.  Wright  wedge. 

Of  these  accessories  1  and  2  accompany  the  simplest  polarizing  micro- 
scopes, while  the  others  are  used  more  for  special  investigations.  Numbers 
5-7  are  generally  placed  in  a  special  ocular  from  which  they  can  be  removed 
by  a  screw.  When  they  are  used,  the  nicol  in  the  tube  must  be  thrown  out, 
which  is  then  replaced  by  an  analyzer  placed  over  the  ocular.  The  others 
are  inserted  in  a  slot  in  the  tube  just  above  the  objective  or  in  a  slot  above 
the  ocular  in  the  older  models  where  the  analyzer  was  placed  on  top.  The 
slots  are  sometimes  arranged  so  that  the  long  edge  of  the  plate  lies  at  45° 
to  the  vibration  directions  of  the  nicols  and  sometimes  so  that  they  are 
parallel.  In  all  cases,  however,  the  vibration  directions  of  the  compensators 
must  form  an  angle  of  45°  with  those  of  the  nicols,  hence  they  are  con- 
structed differently  according  to  the  different  modifications  of  the  instru- 
ments. In  the  first  case  the  vibration  directions  are  parallel  to  the  edges 


Q— 


FIG.  106.— Parallel.  FIG.  107.— Diagonal. 

Compensator  with  Vibration  Direction. 

of  the  rectangle  and  the  direction  of  greatest  or  of  least  elasticity  may  be 
parallel  to  the  long  edge.  Formerly  the  quarter  undulation  plates  were 
nearly  always  made  with  the  direction  of  the  smallest  velocity  corresponding 
to  the  long  edge,  and  the  other  plates  with  reversed  orientation,  Fig.  106. 
At  the  present  time  the  quarter  undulation  plate  is  also  orientated  in  this 
jvay,  in  order  to  obviate  the  great  confusion  which  arises  from  the  use  of 
the  other  plates.  It  is  still  better  to  construct  them  all  as  shown  in  Fig.  107, 
where  the  vibration  directions  form  an  angle  of  45°  with  the  edges  of  the 
plate.  The  direction  of  greatest  velocity  a  is  marked  on  it  with  a  diamond 
point.  It  has  the  added  advantage  in  that  it  is  only  necessary  to  invert  the 
plate  to  bring  about  an  exchange  of  the  principle  vibration  directions  while 
the  crystal  remains  stationary. 

Very  frequently  complete  compensation  cannot  be  obtained  on  account 
of  the  dispersion  of  the  double  refraction  in  different  substances,  which 
may  be  very  troublesome  for  accurate  determinations  of  the  double  re- 


90 


PETROGRAPHIC  METHODS 


fraction.  In  crystals  with  distinct  super-  or  sub-normal  interference  colors, 
measurements  cannot  be  made  very  exactly.  Although  compensators 
can  be  made  from  any  colorless,  double  refracting  material,  gypsum,  mica 
and  quartz  are  used  in  general  for  this  purpose. 

1.  A  quarter  undulation  mica  plate  is  a  thin  cleavage  piece 
of  colorless  mica  which  causes  a  retardation  of  both  rays   of 
about   1/4  wave  length  for  sodium  light,  i.e.,   about  150  /*/*. 
Observed  between  crossed  nicols  it  appears  grayish-blue  of  the 
first  order,  and  between  parallel  nicols  brownish- white.     In  many 
instances  plates  which  cause   a   retardation  of  only  1/8  A  are 
of  good  service.     The  quarter  undulation  mica  plate  is  used  to 
orientate  the  other  compensators.     It  shows  a  biaxial  inter- 
ference figure  in  convergent  light.     Since  it  is  optically  negative 
the  direction  of  the  axial  plane  corresponds  to  the  direction  of 
least  elasticity. 

2.  The  compensators  with  reddish-violet  I,  II,  etc.,  have  al- 
ready been  described  under  the  head  of  stauroscopes.     Their 
additional  use  as  compensators  justifies  repetition  at  this  time. 


FIG.  108. — Mica  Wedge. 


Violet  I  is  generally  used  to  determine  the  character  of  the  double 
refraction,  while  violet  II  and  plates  with  higher  interference 
colors  are  used  with  very  strong  double  refracting  substances. 
They  are  generally  prepared  from  gypsum  or  quartz. 

3.  The  Fedorow  mica  wedge,  Fig.  108,  consists  of  a  pile  of  16 
plates  of  mica  of  equal  thickness.     Each  plate  produces  a  retarda- 
tion of  1/4  ^  and  is  shorter  by  a  definite  amount  than  the  one 
next  under  it.     With  this  apparatus  the  first  four  orders  of 
color  appear  in  16  divisions  and  each  division  can  be  made  s<5 
wide  that  the  field  of  vision  of  the  microscope  shows  a  single 
homogeneous  interference  color  at  any  given  time. 

4.  The  quartz  wedge  is  a  small  plate   of  quartz  cut  in  the 
shape  of  a  wedge  and  the  retardation  at  various  places  is  shown 
by  a  scale.     It  is  inserted  in  the  slot  above  the  objective  until 
complete  darkness  or,  still  better,  the  sensitive  violet  I  is  obtained 
by  means  of  compensation.     The  thickness  of  the  quartz  is  read 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  91 

off  at  this  point,  from  which  the  double  refraction  of  the  crystal 
can  be  calculated. 

5.  The  birefractometer  consists   of   an   ocular  within  which 
a  quartz  wedge  can  be  moved  by  means  of  a  rack  and  pinion.     A 
micrometer  scale  is  etched  upon  the  glass  strip  that  covers  the 
wedge.     From  it  the  thickness  of  that  portion  of  the  wedge  in  use 
can  be  read  directly.     It  is  still  better  to  have  the  quartz  wedge 
so  made  that  only  one  half  of  the  field  of  vision  is  colored,  while 
the  scale  is  placed  upon  the  other  half.     With  this  arrangement 
the  apparatus  can  be  used  not  only  as  a  compensator  but  also 
as  a  comparator  in  which  case  the  crystal  is  placed  in  the  vacant 
half  of  the  field  and  its  interference  color  is  compared  with  the 
color  of  the  movable  quartz  wedge. 

6.  The    Babinet   compensator   consists   of   two   thin   quartz 
wedges  with  equal  refracting  angles,  lying  one  on  the  other,  Fig. 
109.     In  one  wedge  the  optic  axis  is  parallel,  in  the  other  perpen- 
dicular, to  the  sharp  edge.     The 

upper  wedge  is  not  movable  but 
the  lower  one  can  be  moved  by 
a  rack  and  pinion,  and  the 

amount    Of    the     movement     Can  FIG.  109.-Babinet  Compensator. 

be    read    on  a    drum.     In    the 

zero  position  there  is  a  dark  band  in  the  center  of  the  field 
marked  by  an  oblique  cross  hair,  Andreas  cross  hair,  because 
both  wedges  are  of  equal  thickness  at  that  place  and  since  their 
equivalent  directions  are  crossed  their  double  refraction  is  entirely 
compensated.  On  either  side  there  is  a  symmetrical  series  of 
bands  with  an  increasing  order  of  interference  colors.  When  a 
double  refracting  plate  is  inserted  the  dark  band  is  removed  from 
the  middle  of  the  field.  The  movable  wedge  is  then  moved  in  by 
means  of  the  screw  until  the  band  lies  again  exactly  in  the  middle 
and  the  amount  of  displacement  is  read  on  the  drum.  If  the  crystal 
is  rotated  90°  the  black  band  is  removed  toward  the  opposite  side. 
It  is  again  placed  on  the  cross  hair  and  the  mean  of  the  two 
readings  is  indicative  of  the  double  refraction  of  the  crystal. 
An  approximate  adjustment  of  the  central  band  should  be  made 
in  white  light,  because  in  monochromatic  light  dark  bands  occur 
in  place  of  the  colored  ones,  but  the  fine  adjustment  should  be 
consummated  in  sodium  light.  The  double  refraction  of  the  plate 

2  T  tg  tynn 
in  question  can  be  calculated  form  the  formula  y—  a  =  — 


92  PETROGRAPHIC  METHODS 

in  which  r  is  the  amount  of  displacement  for  one  rotation  of  the 
screw,  $  is  the  angle  of  the  wedge  of  the  comparator,  m  is  the 
number  of  rotations  of  the  screw,  and  d  is  the  thickness  of  the 
plate  under  investigation. 

7.  The  twinning  compensator  can  be  described  most  easily  by 
calling  it  a  Babinet  compensator  which  is  cut  through  the  center 
parallel  to  the  direction  of  the  long  arrow  in  Fig.  109.     One  half 
is  rotated  on  the  fresh  cut  through  180°  with  respect  to  the 
other,  and  each  portion  is  left  movable  in  opposite  directions. 
In  the  normal  position  there  is  a  dark  band  across  the  entire 
field  of  vision  corresponding  to  the  complete  compensation  of 
the  double  refraction  in  the  two  oppositely  orientated  portions 
of  the  wedge.     If  a  double  refracting  crystal  is  placed  in  the 

path  of  the  rays  the  band  recedes  just 
as  far  toward  the  right  in  one  half  of  the 
field  as  it  does  toward  the  left  in  the 
other  half,  and  the  amount  of  displace- 

FIG.  110.— Bravais  Twin  Com- 
pensator, ment  necessary  to  unite  both  parts  of 

the  band  again  indicates  very  accurately  the  double  refraction. 
Bravais  constructed  another  type  of  twinning  compensator, 
Fig.  110.  The  two  parts,  rotated  through  180°  to  each  other, 
were  not  placed  side  by  side  but  one  over  the  other.  Then  the 
whole  field  of  vision  is  dark  in  the  normal  position  because  the 
double  refraction  of  the  oppositely  orientated  portions  of  the 
wedge  is  compensated  completely.  When  a  double  refracting 
substance  is  placed  in  the  path  of  the  light,  darkness  is  restored 
by  moving  one  of  the  wedges.  The  displacement  of  the  wedge 
is  indicative  of  the  double  refraction  but  it  is  by  no  means  as 
accurate  as  in  the  previous  case. 

8.  The  Biot  rotating  quartz  is  also  a  very  useful  device  for 
determining  the  character  of  the  double  refraction.     A  plate  of 
quartz  perpendicular  to  the  axis  is  ground  so  thin  that  circular 
polarization  is  no  longer  apparent.     This  is  set  in  a  holder  so 
that  it  can  be  inserted  in  a  slot  above  the  objective  and  rotated 
on  a  horizontal  axis.  -  In  the  normal  position  it  produces   no 
change  but  as  soon  as  it  is  rotated,  the  vibration  direction  of 
the  ordinary  ray  being  the  axis  of  rotation,  it  becomes  double 
refracting.     Any  amount  of  compensation  can  be  obtained  by 
more  or  less  rotation. 

9.  The  simple  wedges  cannot  always  be  used  advantageously 
because  the  lowest  interference  colors  cannot  be  perceived  on 


OBSERVATIONS  IN  PARALLEL  POLARIZED  LIGHT  93 

account  of  the  thinness  of  the  wedge.  Wright  therefore 
fastens  a  gypsum  or  quartz  wedge  on  an  oppositely  orientated 
plane  parallel  plate  of  gypsum  which  gives  violet  I.  By  the 
subtraction  of  one  order  of  colors  the  lowest  colors  of  the  first 
order,  beginning  at  the  zero  point,  are  obtained  instead  of  those 
bordering  violet  I. 

Use  of  Compensators. — To  use  any  of  these  compensators  the  crystal 
under  investigation  is  centered  and  its  vibration  directions  placed  at  45° 
to  those  of  the  nicols.  Except  when  using  a  Babinet  or  a  twinning  com- 
pensator, the  uses  of  which  were  described  above,  the  interference  color  of 
the  object  is  first  observed  and  then  the  compensator  inserted  and  the 
direction  of  change  in  the  interference  color  noted,  i.e.,  whether  the  inter- 
ference color  of  the  combination  corresponds  to  the  sum  or  the  difference 
of  the  retardations  in  the  crystal  and  in  the  compensator.  If,  for  example, 
an  optically  uniaxial  negative  crystal  with  a  prismatic  development  is 
orientated  on  the  stage  of  the  microscope  so  that  its  long  direction  coincides 
with  the  direction  of  greatest  velocity  in  the  compensator,  the  interference 
color  increases.  It  appears  as  if  the  crystal  had  become  thicker.  If  the 
conditions  are  reversed,  the  interference  color  is  lowered  as  if  the  layer 
were  now  thinner.  Those  crystals,  which  show  very  low  interference  colors, 
are  apparently  an  exception  to  this  rule.  For  example,  a  crystal  that 
shows  gray-blue  of  the  first  order,  when  compensated  by  violet  I  in  crossed 
position,  shows  a  brilliant  orange-yellow  interference  color,  which  never- 
theless indicates  subtraction  because  that  is  lower  than  violet  I.  This 
can  be  best  shown  by  rotating  the  crystal  90°.  Then  a  higher  blue  of  the 
second  order  appears.  In  general  it  is  not  known  from  simple  observations 
in  parallel  polarized  light  that  the  crystal  observed  is  uniaxial  and  pris- 
matically  developed.  That  the  direction  of  greatest  velocity  is  parallel  to 
its  long  edge,  its  principal  zone,  is  all  that  can  be  determined.  The  de- 
termination of  the  relative  velocities  of  the  two  rays  vibrating  in  a  cross 
section  is  designated  as  determination  of  the  optical  character  of  the  principal 
zone.  'That  direction  in  which  a  crystal  or  crystal  section  is  elongated  is 
taken  as  the  principal  zone.  The  principal  zone  is  negative  when  the  direc- 
tion of  greatest  elasticity,  and  positive  when  the  direction  of  least  elasticity 
is  parallel  to  the  long  edge. 

The  determination  of  the  optical  character  of  the  principal  zone  is  very 
useful  in  the  determination  of  crystals.  Its  value  can  be  best  appreciated 
for  the  study  of  thin  sections  by  comparing  the  optical  character  of  the 
principal  zone  with  the  true  optical  character  of  the  crystal.  This 
indicates  the  crystallographic  habit,  the  position  of  the  optic  plane,  and  so 
forth. 

The  example  cited  above  shows  that,  in  an  optically  uniaxial  crystal  in 
which  the  principal  axis  corresponds  to  the  principal  zone,  the  optical 
character  of  the  latter  is  the  same  as  the  optical  character  of  the  crystal. 
If  a  uniaxial  negative  crystal  is  tabular  parallel  to  the  base,  a  transverse 
section  will  show  a  decided  elongation  but  the  principal  axis  is  perpendic- 
ular to  it,  i.e.,  the  direction  of  smallest  velocity  is  parallel  to  the  principal 


94 


PETROGRAPHIC  METHODS 


zone  and  its  character  is  positive, 
possibilities. 


The  following  table  shows  the  various 


Character  of  crystal 
+  or  - 
+  or  - 

+  or  - 
+  or  - 
+  or  - 


Uniaxial 

Prismatic  habit 

Tabular  habit 

Biaxial 

Prismatic;  Bx0  |j  principal  zone 
Tabular;  Bx0     J_  tabular  face 
Prismatic;  Bx0  ||  principal  zone 
Tabular;  Bx^j    J_  tabular  face 
Prismatic;  optic  normal  J_ principal  zone 
Tabular;  optic  normal     I    tabular  face 


Character  of  principal  zone 
+  or  -. 
-  or  +. 


-  or  +. 


±. 


The  character  of  the  double  refraction  of  the  principal  zone  varies  in 
different  sections  of  a  biaxial  substance  when  the  vibration  direction  in 
that  section  of  the  zone  is  the  optic  normal. 

The  determination  of  the  character  of  the  principal  zone  is  frequently 
impossible,  either  because  an  actual  characteristic  zone  cannot  be  recognized 
or  because  the  interference  color  is  white  of  the  higher  order.  In  the  former 
case  it  is  impossible  to  speak  of  a  principal  zone,  while  in  the  latter,  other 
methods  must  be  used.  Either  the  wedge-shaped  borders  of  the  crystal 
or  slide  are  used  and  the  ordinary  methods  of  compensation  applied  to  the 
brilliant  bands  of  color,  or  compensators  corresponding  to  a  higher  order  of 
color  can  be  employed,  and  by  subtraction  brilliant  interference  colors  of 
lower  orders  produced. 


CHAPTER  V 
Observations  in  Convergent  Polarized  Light 

Direction  of  the  Rays  in  Convergent  Light. — It  is  important 
in  many  cases  to  study  the  optical  properties  of  a  crystal  or 
crystal  section  not  only  in  one  direction  but  in  as  many  different 
directions  as  possible.  Either  convergent  polarized  light  is  used 
for  this  purpose,  by  means  of  which  the  optical  properties  of  a 
crystal  can  be  observed  in  various  directions  at  one  time,  or  a 
rotating  apparatus,  described  in  the  appendix,  is  used  and  the 
different  directions  studied  in  sequence. 

Fig.  Ill  shows  diagrammatically  the  passage  of  the  rays  in 
convergent  light.  The  lenses  L  and  I/  are  so  arranged  that  the 
upper  focus  of  L  coincides  with  the  lower  focus 
of  L'  at  the  point  /.  The  crystal  K  to  be  investi- 
gated is  placed  between  the  two  lenses.  Each 
of  the  illuminating  cones  of  light  with  its  apex 
in  the  lower  focal  plane  F  of  the  lens  L  is  con- 
verted by  that  lens  into  a  bundle  of  parallel  rays, 
and  these  bundle's  emerge  from  the  lens  with  all 
possible  inclinations  to  the  axis  of  the  lens  within 
its  angle  of  aperture.  These  parallel  rays  pass 
through  the  object  between  the  lenses  and  are 
reunited  in  points  in  the  upper  focal  plane  F'  of 
the  lens  L'. 

Every  point  in  the  image,  thus  produced, 
corresponds  to  a  direction  in  the  crystal.  In 
convergent  light  the  optical  phenomena,  which  an  object  shows 
in  all  the  directions  in  which  light  is  propagated  through  it,  are 
observed  side  by  side.  The  different  retardations  which  the 
light  has  experienced  in  the  various  directions  in  a  crystal 
cannot  be  observed  without  the  aid  of  other  devices,  because 
the  eye  is  not  properly  adjusted  and,  hence,  nothing  at  all  is 
seen  in  convergent  light  until  such  apparatus  is  used. 

If  instead  of  ordinary  light  polarized  light  produced  by  the 
polarizer  P  and  the  analyzer  A  is  used,  interference  phenomena 
become  visible  at  each  point  of  the  image  when  double  refracting 

95 


F — 1~    -i— 

P 
Fia.  111. 

Convergent  Light. 


96 


PETROGRAPHIC  METHODS 


FIG.  112. — Field  of  Vision  in  Convergent 
Light. 


crystals  are  studied.  This  corresponds  to  a  retardation  of  one 
ray  over  the  other  upon  passing  through  the  crystal  in  that 
particular  direction.  The  interference  figure  thus  obtained  is 
one  of  the  most  important  means  of  determining  the  optical 
properties  of  a  crystal. 

It  must  always  be  kept  clearly  in  mind  that  in  observations 
in  convergent  light  any  given  portion  of  the  interference  figure 

does  not  correspond  to  a  definite 
point  on  the  crystal  but  rather  to 
some  direction  in  it.  The  image 
is  not  changed,  if  a  homogeneous 
crystal  is  moved  parallel  to  itself 
on  the  stage.  It  is,  in  general, 
altered  immediately  if  the  direc- 
tion of  observation  is  changed  by 
rotating  the  crystal  on  an  axis. 

The  simple  lenses  L  and  I/  are 
now  replaced  by  combinations  of 
plano-convex  lenses.  The  special 
case  here  considered  is  one  in  which  a  converging  lens  is  placed 
over  the  polarizer  and  L'  is  replaced  by  any  objective. 

The  circle  AB,  Fig.  112,  represents  the  field  of  vision  in  convergent 
polarized  light.  For  the  sake  of  simplicity,  the  angle  of  aperture  may  be 
taken  equal  to  90°.  Rays,  which  have  left  the  crystal  plate  with  an  in- 
clination of  45°  to  the  vertical,  emerge  around  the  outer  edge  of  the  field. 
Assuming  the  index  of  refraction  /?  for  the  crystal  to  be  1.60,  it  follows 

from  the  law  of  sines,  sin  V  = that  the  rays  at  the  edge  of  the  image 

P> 

have  passed  through  the  crystal  at  an  angle  of  26  1  /2°  to  the  vertical,  or  the 
entire  field  includes  a  cone  of  light  in  the  crystal  with  an  angle  of  53°.  If  a 
smaller  circle  is  taken  in  the  field  of  vision,  for  example,  one  with  2/3  the 
diameter,  CD,  it  embraces  all  rays  which  emerge  from  the  crystal  under  an  in- 
clination of  about  28  I/  2°  to  the  vertical  or  pass  through  the  crystal  at  172/3°, 
Fig.  113.  Then,  if  a  circle  with  only  1/3  the  diameter  of  the  field  is  taken,  EF, 
it  includes  those  rays  which  leave  the  crystal  at  an  angle  of  13°,  and  are 
propagated  through  it  under  an  angle  of  about  9°.  Finally,  in  the  center  of 
the  image  O,  Fig.  112,  the  optical  phenomena  correspond  to  those  rays 
which  pass  through  the  crystal  plate  exactly  vertically.  From  the  above, 
it  appears  that  with  an  objective  with  a  definite  angle  of  aperture  the  cone  of 
rays  passing  through  a  crystal  will  be  greater  the  smaller  the  value  of  ft,  the 
denominator  of  the  equation  for  the  law  of  sines.  Feebly  refracting  minerals 
give  a  much  more  extensive  perspective  of  their  optical  properties  than 
those  with  higher  indices  of  refraction.  The  value  of  /?  can  be  greatly 
reduced  by  increasing  the  aperture  by  means  of  an  immersion  system. 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT    97 

Methods  of  Observation  in  Convergent  Polarized  Light. — The 

simplest  method  to  pass  from  the  observation  of  the  object  to 
that  of  the  interference  figure  consists  in  removing  the  ocular 
and  observing  the  interference  phenomena,  RS,  Fig.  113, 
produced  by  the  objective  alone.  It  consists  of  a  small  image 
visible  in  the  upper  focal  plane  of  the  objective.  This  is  the 
Lasaulx  method.  Inversion  of  the  image  does  not  occur  in  this 


FIG.  113. — A  Biaxial  Interference  Figu-e. 

method  of  observation.  The  observed  interference  figure  has 
the  same  position  as  the  object,  but  is  inverted  with  respect  to 
the  microscopic  image.  The  rays  which  have  passed  through 
the  crystal  from  left  below  to  right  above  appear  in  the  image  on 
the  right.  Images  thus  produced  are  generally  very  small  but 
sharply  defined.  They  can  be  magnified  but  a  part  of  the  dis- 
tinctness is  lost  by  the  process.  A  low  power  microscope  can 

7 


98 


PETROGRAPHIC  METHODS 


be  obtained  by  using  the  ocular  A,  Fig.  113,  and  the  Bertrand 
lens  B,  inserted  in  the  tube  as  an  objective.  This  can  be  focused 
on  the  small  interference  figure  by  elongating  the  tube  and  it 
gives  a  magnified  image  R"S",  which  is  observed  in  the  ocular. 
The  image  thus  observed  is  inverted  with  respect  to  the  object 
and  has  the  same  position  as  the  microscopic  image  of  the  object. 
The  proper  centering  of  the  Bertrand  lens  must  always  be  checked  first 
by  placing  a  uniaxial  crystal,  tabular  parallel  to  the  base,  in  the  micro- 
scope. If  the  position  of  the  lens  is  correct,  the  dark  cross,  described  below, 
must  coincide  with  the  cross  hairs  of  the  ocular.  The  interference  figure 
can  also  be  magnified  by  placing  a  Klein  lens  above  the  ocular.  This  lens 
can  be  moved  in  a  vertical  direction  in  its  holder.  The  magnified  image 
in  this  case  is  likewise  inverted  with  respect  to  the  object,  but  its  position 
coincides  with  the  microscopic  image  of  the  object.  A  real  image  R'S', 
adapted  for  projection  and  photography,  is  obtained  by  using  the  Bertrand 
lens  without  the  ocular,  or  by  using  a  projection  ocular.  This  image  is 
likewise  inverted  with  respect  to  the  object. 

The  interference  figure  observed  appears  smaller  the  higher  the 
power  of  the  objective  used.  More  convergent  bundles  of  rays 
are  obtained  from  stronger  objectives,  because  in  general  the 
aperture  of  the  system  increases  with  the  magnification.  The 
interference  figure  obtained  with  a  high  power  objective,  allows 
directions  at  greater  inclinations  to  each  other  to  be  studied, 


II 


(Homogeneous 

Immersion) 

M. 


OOa 


n-\ 


n=l 


w  =  l 


1.515 


2w  =  8°  2w=30°  2w  =  74°  2w  =  128°  2tt  =  118° 

FIG.  114. — Angle  of  Aperture  of  the  most  Important  Objectives  of  W.  and  H.  Seibert. 

provided  the  aperture  of  the  illuminating  apparatus  is  great  enough  for  that 
of  the  objective.  For  observations  in  convergent  light,  the  condenser 
must  always  be  in  position  and  high  power  objectives1  used. 

Cross  sections  of  the  various  objectives  made  by  W.  and  H.  Seibert  are 
shown  in  Fig.  114  in  which  2u  is  the  angle  of  aperture.  Fig.  115  shows 
the  corresponding  interference  figure  of  each  in  an  optically  biaxial  mineral 
with  an  apparent  optic  angle  of  about  85°. 

If  the  aperture  of  the  illuminating  apparatus  is  not  sufficiently  large  for 

1  When  such  objectives  are  not  available  convergent  light  can  be  produced  by  a  small 
air  bubble  in  the  Canada  balsam  or  by  a  soap  bubble,  which  takes  the  place  of  the  lens  O. 
The  interference  figure  so  produced  appears  magnified  in  the  microscope. 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT    99 


the  objective,  only  the  inner  part  of  the  image  will  be  illuminated  and  the 
border  will  appear  dark.  If  one  of  a  large  number  of  minute  individuals 
is  to  be  studied  in  convergent  polarized  light,  or  if  a  crystal  consists  of  many 
small  twinned  individuals,  the  optical  nature  of  which  is  to  be  determined, 
an  ocular  diaphragm  should  be  used.  With  it  only  the  light  passing  through 
that  portion  of  the  object  in  the  center  of  the  field  of  vision  reaches  the  eye. 
The  simplest  method  for  such  observations  is  to  center  the  desired  portion 


OOa, 


FIG.  115. — Interference  Figures  Corresponding  to  the  Objectives  in  Fig.  114. 

of  the  object  carefully  and  upon  removing  the  ocular,  place  a  diaphragm 
with  a  small  perforation  in  its  center  over  the  end  of  the  tube.  The  object 
can  be  centered  by  using  a  Ramsden  ocular  with  an  iris  diaphragm  in  the 
lower  focal  plane.  The  diaphragm  is  narrowed  until  only  the  portion  of  the 
object  to  be  studied  is  visible  in  parallel  light.  Then  the  ocular  is  removed 
while  the  iris  diaphragm  remains  in  the  tube.  In  investigations  by  the 
Bertrand  method  the  iris  diaphragm  is  placed  in  the  upper  focal  plane  of  the 
lens  which  is  inserted  into  the  tube. 

Optically  Uniaxial  Crystals. — Let  us  consider  more  closely  the 
behavior  in  convergent  polarized  light  of  a  plate  of  a  uniaxial 
crystal  cut  perpendicular  to  the 
optic  axis.  One  of  the  several 
bundles  of  parallel  rays  AA',  Fig. 
116,  passes  through  the  plate  per- 
pendicularly and  is  parallel  to  the 
optic  axis.  It  is  at  the  same  time 
the  axis  of  the  cone  of  light.  There 
is  no  double  refraction  in  the  direc- 
tion of  the  optic  axis  so  the  center 
of  the  interference  figure  will  ap- 
pear dark.  If  a  second  bundle  of 
rays  A'w,  which  passes  through 
the  crystal  slightly  inclined  to  its 

axis,  is  considered,  the  light  will  be  resolved  into  two  rays,  one 
vibrating  in  the  principal  section  of  the  crystal  and  the  other 
perpendicular  to  it.  These  rays  will  emerge  from  the  crystal 
with  a  small  phasal  difference  corresponding  to  the  small  inclina- 


FIG.  116. — Construction  of  an  Uniaxial 
Interference  Figure. 


100  PETROGRAPHIC  METHODS 

tion  to  the  axis,  and  weak  illumination  will  appear  at  that  point. 
Other  bundles  of  rays  A'Z  pass  through  the  crystal  more  inclined 
to  the  axis  and  have  a  decidedly  greater  retardation  correspond- 
ing to  this  greater  inclination.  Thus,  there  is  a  continual  increase 
of  interference  color  from  the  center  toward  the  edge  of  the 
image.  All  rays  with  equal  inclination  to  the  axis  have  the  same 
retardation  so  the  interference  colors  appear  in  concentric  rings 
around  the  dark  middle  point  in  the  order  shown  in  Fig.  96. 
They  are  most  brilliantly  illuminated  in  those  sections  forming  an 

angle  of  45°  with  the  vibration 
directions  of  the  two  nicols.  As 
light  is  not  resolved  in  those 
sections  coinciding  with  the 
vibration  directions  of  the  nicols, 
they  remain  dark  under  all  con- 
ditions. Therefore,  the  inter- 
ference figure  consists  of  a  dark 
cross,  the  arms  of  which  are 
parallel  to  the  vibration  direc- 
tions of  the  nicols  and  cut  across 
the  colored  concentric  rings, 

FIG.  117.— Interference  Figure  of  a  high      ^  *S'  ' 

Double  Refracting  Uniaxial  Crystal.  All     directions      perpendicular 

to  the  optic  axis  are  equal   in 

uniaxial  crystals  so  there  is  no  change  in  the  image  when  the 
stage  is  rotated  through  360°,  and  all  these  directions  coincide 
in  turn  with  the  vibration  directions  of  the  nicols.  Interference 
figures  of  optically  uniaxial  crystals  cut  perpendicular  to  the 
optic  axis  are  not  changed  by  a  complete  horizontal  rotation  of 
the  section. 

The  behavior  of  uniaxial  crystals  in  sections  perpendicular  to  the  optic 
axis  between  parallel  nicols  can  be  understood  from  the  above  discussion. 
Light  is  not  resolved  in  that  principal  section  of  the  crystal  which  corre- 
sponds to  the  vibration  direction  of  one  of  the  nicols  and  in  the  direction 
perpendicular  to  it.  A  light  cross  appears  instead  of  the  dark  one,  de- 
scribed above,  cutting  across  the  concentric  colored  rings,  whose  colors  are 
complementary  to  those  obtained  between  crossed  nicols. 

If  the  face  through  which  the  observation  of  the  interference  figure  is 
made  is  not  exactly  perpendicular  to  the  optic  axis,  the  rays  observed  in 
the  center  of  the  field  are  not  those  which  pass  through  the  crystal  parallel 
to  the  optic  axis.  The  intersection  of  the  arms  of  the  dark  cross  is  dis- 
placed from  the  center  of  the  field  and  the  colored  rings,  the  center  of  which 


OBSERVATIONS,  CONVERGENT  POL  ARPZED*  LIGHT it)! 


is  this  intersection  point,  are  eccentric  with  respect  to  the  field  of  vision. 
When  the  slide  is  rotated,  the  intersection  of  the  cross  arms  describes  a 
circle  about  the  center  of  the  field.  See  the  upper  series  in  Fig.  118.  The 
directions  of  the  arms  of  the  cross  are  dependent  only  upon  the  vibrations 
of  the  nicols  and,  since  these  are  not  changed,  the  arms  retain  their  directions 
but  are  displaced  across  the  field  parallel  to  the  original  positions. 

If  the  section  is  cut  so  obliquely  that  w  sin  /JL  >  u,  in  which  w  is  the  index 
of  refraction  of  the  ordinary  ray  in  the  crystal,  /*  is  the  angle  of  inclination 
of  the  section  to  the  optic  axis,  and  u  is  the  aperture  of  the  ocular,  and  the 
apparent  direction  of  the  optic  axis  falls  outside  of  the  angle  of  aperture  of 
the  ocular,  a  comparatively  reliable  clue  can  still  be  obtained  that  the 
crystal  is  uniaxial.  As  shown  in  the  lower  series  of  Fig.  118,  the  extreme 


FIG.   118. — Interference  Figures  of  Uniaxial  Crystals  Cut  Cblique  to  the  Axis. 

portions  of  the  black  cross  pass  through  the  field  of  vision  in  a  regular  man- 
ner when  the  crystal  plate  is  rotated.  The  difference  between  interference 
figures  in  oblique  sections  of  uniaxial  crystals  and  those  of  biaxial  crystals 
is  emphasized  in  Fig.  118,  in  that  the  black  bars  which  pass  across  the  field 
are  represented  exactly  parallel  to  the  vibration  directions  of  the  two  nicols. 
This  is  not  the  case  when  the  axis  emerges  very  obliquely  and  the  aperture 
of  the  objective  is  quite  large.  Then,  as  they  are  shifted,  their  directions 
are  changed  very  decidedly  just  a.s  in  a  biaxial  interference  figure.  But 
sections  of  biaxial  crystals  with  small  cptic  angles,  cut  not  too  obliquely, 
show  perfectly  parallel  shifting  of  the  bars,  particularly  when  objectives 
with  small  apertures  are  used.  It  may  also  be  noted  that  the  emergence  of 
the  optic  axis  from  a  cleavage  fragment  of  calcite,  where  w  sin  fj.  =  1.165,  can 
only  be  se.en  by  means  of  an  immersion  system. 

The  behavior  of  a  plate  of  a  uniaxial  crystal  cut  parallel  to  the  optic  axis 
in  a  rock  section  cannot  be  distinguished  in  ordinary  light  from  that  of  a 
biaxial  crystal  cut  perpendicular  to  its  obtuse  bisectrix.  This  will  be 
treated  more  fully  farther  on.  It  is  impossible  to  distinguish,  by  simple 
means,  a  section  of  a  uniaxial  crystal  parallel  to  the  optic  axis  from  a  bi- 
axial. The  greater  the  inclination  of  tne  plate  to  the  optic  axis  the  more 
positive  the  determination.  However,  the  direction  of  the  optic  axis  or  that 
of  the  acute  bisectrix  can  be  detei  mined  in  the  interference  figure  in  a 
biaxial  crystal  with  not  too  large  an  optic  angle.  Let  us  assume  that  the 


102  'P&TiiOGRAPHIC  METHODS 

cross  section  given  in  Fig.  119  is  parallel  to  the  optic  axis  cc  of  a  crystal, 
which  is  observed  in  convergent  light  at  45°  to  the  vibration  directions  of 
the  nicols  and  the  characteristic  distribution  of  the  interference  colors  is 
seen.  The  observed  phenomena  can  be  best  shown  in  a  plate  cut  from  the 
section  parallel  to  its  long  direction,  Fig.  120,  and  one  perpendicular  to  it, 
Fig.  121.  The  simplest  explanation  can  be  given  with  the  assumption  that 
the  angle  of  aperture  of  the  objective  is  180°,  which  cannot  actually  be 
obtained  in  practice  but  can  be  approached  approximately.  Ifthebehav- 


FIG.  119. — Section  of  an  FIG.  120.  FIG.  121. 

Uniaxial  Crystal  Parallel  Section  through  Fig.  119. 

to  the  Optic  Axis.  Parallel  to  cc.  Parallel  to  aa. 

ior  of  the  rays  in  the  section  cc  perpendicular  to  Fig.  119,  as  shown  in  Fig. 
120,  is  studied,  it  will  be  seen  that  the  rays  whose  image  appears  in  the 
center  of  the  field  of  vision  have  passed  through  the  crystal  perpendicular  to 
its  optic  axis.  They  interfere  with  the  full  value  of  the  double  refraction 
of  the  crystal  f-a.  The  rays,  however,  which  appear  on  the  extreme 
edge  of  the  field,  have  passed  through  the  crystal  in  the  direction  cc,  i.e.,  the 
direction  of  the  optic  axis,  and  have  suffered  no  double  refraction  whatever. 
In  those  quadrants  through  which  the  optic  axis  passes  the  interference 


\ 


FIG.  122.— Uniaxial  Crystal  Parallel  to  the  Optic 
Axis  in  Convergent  Polarized  Light. 

color  is  lowered  toward  the  edge  of  the  field  but  it  is  not  reduced  to  zero 
as  it  would  be  in  the  theoretical  case  under  consideration.  Red  I,  in  the 
middle  of  the  field,  changes  into  a  distinct  yellow  as  shown  in  Fig.  122.  In 
the  section  given  in  Fig.  121 -the  rays  in  the  middle  of  the  image  are  likewise 
propagated  perpendicular  to  the  optic  axis  and  interfere  with  the  greatest 
double  refraction.  The  rays  in  the  outer  portion  of  the  image  have  the  same 
direction  of  propagation  and  hence  the  same  double  refraction.  The  dis- 
tance traversed  by  the  rays  increases  with  their  obliquity,  causing  an  increase 
in  the  retardation  and  consequently  a  raising  of  the  interference  colors,  red 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  103 


I  passing  over  into  blue.     The  section  of  quartz  cut  parallel  to  the  optic 
axis  may  serve  to  illustrate  this  as  shown  in  the  following  table: 


I. 


II. 


u 

P 

d 

w 

d 

w 

0° 

1.0000 

0.0091 

0.0091 

0.0091 

0.0091 

20° 

1.0652 

0.0069 

0.0073 

0.0091 

0.0095 

40° 

1.3054 

0.0038 

0.0050 

0.0091 

0.0119 

60° 

2.0000 

0.0010 

0.0020 

0.0091 

0.0182 

u  is  the  angle  of  inclination  of  the  rays  toward  the  normal  to  the  section,  p 
is  the  distance  traversed  in  the  direction  indicated,  d  the  amount  of  double 
refraction  in  this  direction,  and  w  the  retardation  of  the  two  rays  resulting 
from  the  last  two  factors.  The  columns  under  I  give  the  value  for  those 
directions  which  lie  in  the  section  corresponding  to  Fig.  120,  while  those 
for  the  section  corresponding  to  Fig.  121  are  to  be  found  in  the  columns 
under  II. 

The  interference  figures  of  uniaxial  minerals  are  often  affected  by  inclu- 
sions, twinning  lamellae,  etc.  When  disturbing  inclusions  are  p.resent  the 
interference  figure  is  undistorted  only 
when  the  vibration  directions  of  the 
inclusions  correspond  to  those  of  the 
nicols.  In  other  cases  the  dark  cross 
in  the  interference  figure  does  not 
remain  unaltered  upon  rotating  the 
object,  but  it  opens  giving  rise  to 
two  rather  irregular  hyperbolas  that 
reunite  on  further  rotation.  Such 
minerals  behave  like  biaxial  crystals 
with  a  very  small  optic  angle.  The 
latter  phenomenon  is  very  common 
so  that  positive  determinations  can- 
not always  be  made.  It  may  occur 
that  the  black  cross  appears  open 
normally,  and  does  not  clo~se  upon 
rotating  the  object.  This  is  caused 

by  double  refraction  in  the  lenses  and  a  normal  image  can  frequently  be 
produced  by  rotating  the  objective. 

In  very  thin  plates  of  low  double  refracting  substances  the  bundles  of 
rays  inclined  most  to  the  optic  axis  do  not  suffer  sufficient  double  refraction 
to  produce  brilliant  interference  colors.  Illumination  occurs  only  at  the 
extreme  border  of  the  field  in  the  four  sectors  lying  between  the  principal 
sections  of  the  nicols.  The  greater  part  of  the  image  is  a  broad,  poorly  de- 
fined cross,  Fig.  123.  For  recognizing  very  weak  double  refraction  in  con- 
vergent light,  a  sensitive  tint  plate  may  be  used.  When  it  is  inserted  the 
interference  figure,  which  was  formerly  scarcely  visible,  becomes  distinct  by 
a  blue  color  in  two  opposite  quadrants  and  orange  in  the  other  two.  If  on 
the  other  hand,  the  plate  is  thick  or  the  substance  has  a  very  high  double 


FIG.  123. — Interference  Figure  of  a  low 
Double  Refracting  Uniaxial  Crystal. 


104  PETROGRAPHIC  METHODS 

refraction,  the  rings  are  very  close  and  the  color  is  of  a  lower  order  in  the 
center,  but  gives  way  rapidly  to  white  of  a  higher  order  toward  the  edge.  A 
distinct  interference  figure  can  be  obtained  even  with  a  low  power  objective. 
Crystals  with  Circular  Polarization.— It  is  not  necessary  to  go  very  deeply 
into  the  phenomenon  of  circular  polarization  in  convergent  light  because  it 
can  only  be  observed  distinctly  in  much  thicker  slides  than  are  used  in  ordi- 
nary microscopic  studies.  Circular  polarization  is  the  property  possessed 
by  certain  bodies,  whereby  light  is  resolved  into  its  components  in  that 
direction  in  which  double  refraction  does  not  usually  take  place.  Since  all 
directions  perpendicular  to  this  one  are  equivalent,  resolution  of  the  light 
can  only  give  rise  to  circular  vibrations  which  move  in  opposite  directions 
and  possess  slightly  different  velocities.  Circular  polarization  can  take 
place  in  isotropic  crystals  in  all  directions  but  in  double  refracting  crystals 
only  in  the  direction  of  an  optic  axis.  Upon  emerging  from  the  crystal  the 
two  circular  vibrations  unite  in  a  plane  polarized  vibration,  the  plane  of 
which  is  rotated  somewhat  corresponding  to  the  retardation  of  one  ray  over 
the  other.  Circular  polarizing  crystals  rotate  the  plane  of  polarization  of 
light 

When  a  circular  polarizing  crystal  section  is  observed  in  parallel,  mono- 
chromatic light,  it  appears  light  between  crossed  nicols,  if  it  is  sufficiently 
thick,  and  there  is  no  change  in  its  luminosity  when  the  plate  is  rotated 

horizontally.  When  one  of  the  nicols 
is  rotated  through  a  definite  distance, 
the  section  becomes  dark.  Depend- 
ing upon  whether  this  rotation  is  to 
the  right  or  to  the  left,  crystals  are 
classified  into  right-  or  left-handed 
crystals,  and  the  amount  of  rotation 
of  the  nicol  necessary  to  produce 
complete  darkness  indicates  the 
strength  of  the  circular  polarization, 
provided  the  thickness  of  the  plate  is 
known.  The  rotation  of  the  plane  of 
polarization  is  generally  quite  differ- 
ent for  different  colors,  e.g.,  in  quartz 

FXG.  124-Interferenc7Figure  of  a  Uniaxial     extreme  violet  is  rotated  21/2   times 

Crystal  with  Circular  Polarization.  as  much  as  extreme  red.     If  a  plate 

cut  perpendicular  to  the  optic  axis  is 

thick  enough,  it  will  show  interference  colors  in  parallel  polarized  light, 
and  these  colors  remain  unchanged  during  a  complete  rotation  of  the 
plate.  When  one  nicol  is  rotated,  the  colors  change  in  such  a  manner 
that  the  various  colors  of  the  spectrum  are  extinguished  in  order,  the 
others  uniting  to  form  a  mixed  color.  Upon  rotating  the  nicol  in  the 
direction  of  the  hands  of  the  clock,  these  complementary  colors  follow  in  the 
order  of  the  spectrum  for  a  right-handed  crystal,  and  in  the  reversed  order 
for  one  which  is  left-handed. 

The  phenomena  of  a  uniaxial,  circular  polarizing  crystal  can  be  very 
easily  deduced  form  the  above.  An  interference  color  pioduced  by  circular 
polarization  appears  in  the  center  of  the  image  and  the  dark  cross  disappears, 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  105 

Fig.  124.  The  color  is  constant  throughout  the  whole  of  the  center  of  the 
field  and  behaves  exactly  as  the  interference  color  of  the  plate  did  in  parallel 
polarized  light.  This  phenomenon  is  generally  not  seen  in  microscopic 
preparations  because  circular  polarization  is  nearly  always  too  weak  to 
produce  an  illumination  which  is  observable  in  a  thin  layer. 

Character  of  the  Double  Refraction  of  Uniaxial  Crystals.— 

In  observing  the  interference  figure  of  a  uniaxial  crystal  it 
was  seen  that  the  phenomenon  was  produced  by  double  refraction 
of  the  rays  passing  obliquely  through  the  plate.  The  light  is 
resloved  into  two  vibrations 'corresponding  to  the  extraordinary 


FIG.   125. — Vibration  Directions  in  an  Uniaxial  Interference  Figure. 

and  ordinary  rays.  The  extraordinary  ray  vibrates  in  a  prin- 
cipal section  and,  since  all  planes  through  the  optic  axis  are  prin- 
cipal sections,  all  radii  of  the  interference  figure  are  vibration 
directions  of  the  extraordinary  ray.  The  tangents'  which  are 
perpendicular  to  the  radii  are  the  vibration  directions  of  the 
ordinary  ray,  Fig.  125. 

If  one  of  the  compensators  already  described,  e.g.,  violet  I,  is 
inserted  in  the  microscope  so  that  its  vibration  directions  are  at 
45°  to  those  of  the  nicols,  the  directions  of  greatest  and  least 
elasticity  in  the  plate  will  be  parallel  to  a  principal  section  of  the 
crystal.  A  reddish-violet  color  appears  instead  of  the  dark  cross, 
as  indicated  in  Fig.  126,  while  the  corners  of  the  four  sectors  are 
colored  yellow  and  blue.  A  negative  crystal  is  represented  in 
Fig.  126.  Its  extraordinary  ray  is  the  faster,  or  the  velocity 


106 


PETROGRAPHIC  METHODS 


of  the  ray  vibrating  in  the  principal  section  or  in  the  radius  of  the 
image  is  the  greater,  and  is  equal  to  a.  Addition  of  the  double 
refraction  takes  place  in  those  sectors  through  which  the  vibration 
direction  of  the  faster  ray  in  the  compensator  passes  radially,  and 
a  higher  interference  color  appears.  It  is  the  brilliant  blue  follow- 
ing violet  I.  In  the  other  sectors  the 
vibration  direction  of  the  slower  ray  in 
the  compensator  is  parallel  to  the  radius 
of  the  interference  figure  and  compensa- 
tion takes  place.  The  interference  color 
is  lowered  in  these  two  sectors,  i.e.,  it 
changes  to  yellow. 

The  simple  mnemonical  rule  for  this 
phenomenon  is:  The  line  joining  the 
blue  sectors  and  the  direction  of  greatest 
velocity  in  the  test  plate  form  a  minus  sign 

for  optically  negative  crystals.  In  positive  crystals  the  other  sectors 
show  the  blue  interference  color.  The  line  joining  the  blue  sectors 
and  the  direction  of  greatest  velocity  in  the  test  plate  form  a  plus 
sign  for  optically  positive  crystals. 

A  quarter  undulation  plate  gives  characteristic  reactions  in  a 
similar  manner.     The  dark  cross  breaks  up  into  two  black  spots 


FIG.  126. — Interference  Figure 
of  a  Negative  Uniaxial  Crystal 
with  Gypsum  Test  Plate. 


•Q' 


p' 


P' 


FIG.  127.— Positive  FIG.  128.— Negative 

Uniaxial  Interference  Figure  with  a  Mica  Test  Plate. 

and  the  line  joining  them  corresponds  to  a  vibration  direction  in 
the  test  plate.  It  is  parallel  to  the  direction  of  greatest  velocity 
in  the  test  plate  for  positive  crystals,  Fig.  127,  and  perpendicular 
to  it  for  negative  crystals,  Fig.  128.  The  colored  rings  may  be 
observed  to  move  out  in  those  two  quadrants  and  move  in  in  the 
other  two. 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  107 

Figs.  127  and  128  show  diagrammatically  the  conditions  described  above. 
The  phenomenon  is  based  on  the  direction  of  greatest  velocity  of  light  in 
the  mica  plate,  but  if,  as  is  frequently  the  case,  the  direction  of  least  elasticity 
is  parallel  to  the  long  direction  of  the  test  plate,  the  two  images  must  be 
interchanged. 

The  eighth  undulation  plate  likewise  gives  a  characteristic  reaction  in 
which  the  dark  cross  breaks  up  into  two  curves  similar  to  the  middle  portion 
of  a  figure  8.  The  figure  is  closed  on  both  ends  by  close  colored  rings  and 
lies  parallel  to  the  direction  of  greatest  velocity  in  the  mica  when  the  crystal 
is  negative,  and  in  the  reverse  position  when  it  is  positive. 

The  investigation  of  the  character  of  the  double  refraction  of  a  thin,  feebly 
double  refracting  mineral  with  a  quarter  undulation  mica  test  plate  between 
crossed  nicols  does  not  give  very  positive  results,  because  the  dark  spots  as 
well  as  the  rings  fall  outside  of  the  field  of  vision.  The  violet  I  plate  is 
better  in  such  cases.  Its  brilliant  color  reaction  is  distinct  even  with  the 
feeblest  double  refracting  substances.  Great  precautions  are  necessary  in 
the  investigation  of  the  weakest  double  refracting  crystals  with  violet  I 
because  the  object  glass  itself  often  gives  a  very  weak  interference  figure 
which  becomes  quite  distinct  with  the  sensitive  tint.  A  quarter  undulation 
plate  is  sufficient  in  almost  all  cases  if  the  observations  are  made  between 
parallel  instead  of  crossed  nicols.  The  four  angles  of  the  brownish  cross, 
which  now  appears,  are  alternately  white  and  deep  brown  passing  over  into 
blue.  The  two  latter  quadrants,  which  in  negative  crystals  are  parallel, 
are  in  positive  crystals  crossed  to  the 
direction  of  greatest  velocity  in  the 
mica  plate,  and  can  be  easily  recog- 
nized by  their  color.  A  mica  test 
plate  gives  the  best  results  with  very 
strongly  double  refracting  substances 
in  which  the  innermost  colored  ring 
is  very  small,  and  also  with  highly 
colored  substances. 

Biaxial  Crystals. — If  a  plate  of 
a  biaxial  crystal  with  not  too 
small  an  optic  angle  is  cut  per- 
pendicular to  an  optic  axis  and 

Observed    in    Convergent     polar-    Fla-  ISO.— Interference  Figure  of  a  Biaxial 
,    , .    ,  .  ..  Crystal  Perpendicular  to  an  Optic  Axis. 

ized  light,  an  image  similar  to 

that  of  a  uniaxial  crystal  will  be  seen.  There  is  only  one  bar  in 
place  of  the  dark  cross  and  nearly  circular  ovals  replace  the  truly 
circular  rings  of  the  uniaxial  figure,  Fig.  129.  The  black  bar  is 
parallel  to  the  vibration  direction  of  one  of  the  nicols,  when  the 
plane  of  the  optic  axes  lies  parallel  to  it,  i.e.,  when  the  only 
principal  section  that  can  be  placed  through  the  plate  is  parallel 
to  the  vibration  direction  of  a  nicol.  When  the  plate  is  rotated 
the  bar  with  the  rings  rotates  also,  but  in  the  opposite  direction, 


108  PETROGRAPHIC  METHODS 

about  a  point  corresponding  to  the  place  of  emergence  of  the 
optic  axis.  After  a  rotation  of  90°  the  bar  is  parallel  to  the 
vibration  direction  of  the  other  nicol.  The  vibration  directions 
of  the  two  nicols  always  form  the  bisectors  of  the  angles  between 
the  dark  bar  and  the  direction  of  the  plane  of  the  optic  axes. 
The  bundle  of  rays  passing  through  the  crystal  parallel  to  the 
optic  axis  suffers  no  double  refraction  in  this  case  and  the 
corresponding  portion  of  the  image  is  therefore  dark.  The  point 
at  which  the  axis  emerges  is  the  pivot  for  the  black  bar  and  is  the 
only  portion  of  the  interference  figure  that  always  remains  dark. 


FiG.»30.  FIG.  131. 

Interference  Figures  of  Biaxial  Crystals  Perpendicular  to  Acute  Bisectrix. 

A  plate  of  an  orthorhombic  crystal  cut  perpendicular  to  the 
acute  bisectrix  shows  the  image  represented  in  Fig.  130,  when  the 
plane  of  the  optic  axes  is  parallel  to  the  prinicpal  section  of  one 
of  the  nicols.  The  figure  shows  a  black  cross,  corresponding  to 
the  two  principal  sections  that  can  be  placed  through  the  plate. 
That  bar  of  the  cross,  which  is  parallel  to  the  axial  plane,  is 
sharply  defined.  The  other  bar,  at  right  angles  to  it,  is  broad 
and  indistinct.  This  cross  cuts  the  system  of  lemniscates  sym- 
metrically. The  inner  curves  are  closed  ovals  while  the  outer 
ones  unite  to  form  a  figure  8.  The  axes  themselves  emerge  in 
the  vertices  of  these  curves.  When  the  plate  is  rotated  the  cross 
opens,  the  arms  uniting  to  form  a  curve  which  is  very  nearly  an 
hyperbola  when  the  plate  has  been  rotated  through  45°,  Fig.  131. 
The  colored  rings  rotate  also  with  the  plate -but  their  form  is  not 
changed.  The  only  points  which  always  remain  dark  when  the 
plate  is  rotated  are  the  points  where  the  axes  emerge,  and  they 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  109 

form  the  vertices  of  the  hyperbola.  The  distance  between  the 
vertices  when  the  axial  plane  is  at  45°  to  the  nicols  indicates  the 
size  of  the  optic  angle. 

The  interference  figure  of  biaxial  crystals  with  very  small  optic  angles 
approximates  that  of  uniaxial  crystals.  The  black  cross  appears  to  be  sur- 
rounded by  a  single  system  of  colored  circular  curves,  Fig.  132,  and  it  opens 
only  slightly  upon  rotating  the  slide.  As  already  remarked,  this  is  some- 
times seen  in  uniaxial  crystals  so  that  a  definite  distinction  between  uni- 
axial and  biaxial  substances  may  become  quite  difficult. 

If  the  optic  angle  is  very  large,  Fig.  133,  it  may  happen  that  the  axes  do 
not  emerge  •within  the  field  of  vision  even  when  the  strongest  objectives  are 


FIG.  132.  FIG.  133. 

Interference  Figure  of  a  Biaxial  Crystal  with  a  very 
Small  Optic  Angle.  Large  Optic  Angle. 

used.  That  is  the  case  when  ft  sin  V>A,  aperture  of  the  objective  used. 
If  ft  sin  V>  1,  the  optic  axes  do  not  emerge  from  the  crystal  into  the  air  at 
all,  because  then  total  reflection  of  the  light  takes  place,  and  the  apparent 
optic  angle  is  greater  than  180°.  If  an  immersion  system  is  used  with  such 
crystals,  the  optic  axes  can  be  observed  in  all  cases  in  a  section  perpendicular 
to  the  acute  bisectrix,  because  then  the  difference  between  the  indices  of 
the  crystal  and  the  medium  surrounding  it  is  decreased,  whereby  the  refrac- 
tion of  the  rays  emerging  from  the  crystal  into  the  immersion  liquid  is 
reduced,  and  the  angle  of 'total  reflection  increased. 

With  an  immersion  system  the  two  axes  can  often  be  seen  in  the  field  of 
vision  in  a  section  cut  perpendicular  to  the  obtuse  bisectrix  if  the  index  of 
refraction  of  the  crystal  is  not  too  great,  and  the  optic  angle  is  very  large. 

Dispersion  of  the  Optic  Axes. — The  optic  angle  is  different  for 
different  colors  and  is  sometimes  larger  for  red  than  for  blue, 
Fig.  134.  It  is  indicated  by  the  dispersion  formula  p>v,  or  the 
reverse  may  be  the  case,  v>  p.  Dispersion  can  scarcely  be  recog- 
nized in  the  microscope  if  it  is  very  slight,  especially  when  the 


no 


PETROGRAPHIC  METHODS 


FIG.  134.— Axial  Plane  in  an 
Orthorhombic  Crystal. 


substance  has  very  low  double 
refraction.  If  the  dispersion  is 
greater,  there  will  be  brilliantly 
colored  bands  around  the  vertices 
of  the  hyperbola  in  the  interfer- 
ence figure  and  this  band  is 
broader  and  more  brilliant  the 
stronger  the  dispersion.  Very 
strong  dispersion  of  the  optic 
axes  can  be  noted  even  in  parallel 
polarized  light  in  the  so-called 
dispersion  colors,  see  page  79. 
If  the  hyperbolas  are  yellow  on 
the  convex  side,  and  blue  on  the 
concave,  the  optic  angle  for  blue 


is  smaller  than  that  for  red, 
Fig.  135.  Those  rays  which  pass 
through  a  crystal  parallel  to  an 
optic  axis  for  a  definite  color  suffer 
no  double  refraction  for  that  color, 
and  the  complementary  color  ap- 
pears in  its  place  in  the  interfer- 
ence figure,  i.e.,  in  the  vertex  of  FlG'  135--°rthoAombi' 

the  hyperbola. 

In  the  orthorhombic  system  the 
bisectrices  and  the  optic  normal  for 
all  colors  coincide  with  the  crystal- 
lographic  axes,  Fig.  134.  One  of 
the  axes  can  be  the  optic  normal  for 
one  color  and  another  axis  for 
another  color,  i.e.,  the  planes  of  the 
optic  axes  for  different  colors  can  be 
crossed,  as  for  example,  in  brookite. 
In  any  case  the  interference  figure 
is  symmetrical  to  the  plane  of  the 
optic  axes  and  the  plane  perpendicu- 
lar to  it.  In  the  monoclinic  system 
in  which  only  the  b  axis  coincides 
with  a  principal  vibration  direction, 
FIG.  i36.-Axiai  Plane  in  a  Monoclinic  symmetry  with  respect  to  one  or 
Crystal  with  inclined  Dispersion,  both  of  these  planes  may  be  lacking. 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  111 


There  are  three  different  kinds  of  dispersion  in  the  monoclinic 
system. 

1.  Inclined  dispersion,  the  b  axis  is  the  optic  normal,  Fig.  136. 
The  optic  axes  lie  anywhere  in  a  plane  parallel  to  the  clinopina- 
coid  and  the  two  axes  suffer  different  amounts  of  dispersion. 


FIG.  137. 
Inclined  Dispersion. 


FIG.  138. — Optic   Plane  in  a  Monoclinic  Crystal 
with  Crossed  (Horizontal)  Dispersion. 


The  interference  figure  is  symmetrical  with  respect  to  the  plane 
of  the  optic  axes  but  not  to  the  plane  perpendicular  to  it,  because 
the  bands  of  color  are  different  on  each  arm  of  the  hyperbola, 
Fig.  137. 

2.  Horizontal  dispersion,  the  b  axis  is  the  obtuse  bisectrix, 


FIG.  139. — Horizontal  Dispersion.  FIG.  140. — Crossed  Dispersion. 

Fig.  138.  (The  bisectrices  I  and  II  must  be  interchanged.) 
The  planes  of  the  optic  axes  for  different  colors  are  any  planes 
perpendicular  to  the  clinopinacoid.  The  interference  figure  is 
not  symmetrical  with  respect  to  the  plane  of  the  optic  axes  but 
to  the  plane  perpendicular  to  it,  Fig.  139. 


112 


PETROGRAPHIC  METHODS 


3.  Crossed  dispersion,  the  b  axis  is  the  acute  bisectrix,  Fig.  138. 
The  position  of  the  plane  of  the  optic  axes  is  the  same  as  in  the 
previous  case.  In  the  interference  figure  the  distribution  of 

color  to  the  right  above  is  the  same 
as  to  the  left  below,  Fig.  140,  that  is, 
the  colors  are  symmetrical  to  a  point. 
As  there  is  no  relation,  in  the 
triclinic  system,  between  the  optic 
directions  for  the  different  colors  and 
the  crystallographic  axes,  the  distri- 
bution of  the  axes  as  well  as  of  the 
planes  of  the  optic  axes  is  entirely 
unsymmetrical,  Fig.  141. 

In  general  these  differences  of  dis- 
persion are  quite  small  and  are 
observed  only  in  very  favorable 
cases.  The  presence  of  distinct  in- 
clined or  crossed  dispersion  must  be 
considered  as  characteristic  of  a 
monoclinic  crystal,  but  the  absence 
of  this  phenomenon  is  not  sufficient 
to  determine  the  crystal  as  ortho- 
rhombic. 


FIG.  141.— Optic  Plane  in  a 
Triclinic  Crystal. 


The  presence  of  strong  dispersion  of  the  optic  axes  is  a  good  earmark  for 
a  substance.     In  monoclinic  crystals  the  character  and  strength  of  the  dis- 
persion are  not  definitely  related  to  each  other.     Titanite,   for  example, 
shows   very  strong  dispersion  of  the   optic 
axes  but  the  inclined  character  is  scarcely 
noticeable.     On  the  other  hand  in  a  certain 
group   of  pyroxenes,   showing  inclined   dis- 
persion, one  axis  is  very  weakly  dispersed, 
the  other  very  strongly. 


Measurement  of  the  Optic  Angle. — 
The  determination  of  the  size  of  the 
optic  angle  is  of  some  importance 
and  can  be  carried  out  by  various 
methods.  The  distance  between  the 


——  w- 

A 

A 

/}'  -1.015 

FIG.  142.     Apparent  Optic  Angle. 


hyperbolas  gives  only  the  size  of  the 

apparent  optic  angle,  which  differs  most  from  the  true  angle, 
the  larger  the  latter,  and  the  higher  the  intermediate  index  of 
refraction  of  the  crystal.  Those  rays  which  pass  through  the 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  113 

plate  parallel  to  the  optic  axes  impinge  upon  the  upper 
surface  of  the  crystal  obliquely  and,  emerging  from  a  stronger 
to  a  lower  refracting  medium,  are  refracted  away  from  the 
normal,  Fig.  142.  Thus,  the  apparent  optic  angle  in  air  is 
always  larger  than  the  true  optic  angle.  If  2E  is  the  apparent, 

sin    E 
and  2V  the  real  optic  angle,  sin  V  =  — - — .    Thus  the  real  optic 

angle  of  a  crystal  can  be  calculated  when  /?  is  known.  Diopside 
and  orthoclase,  for  example,  have  very  nearly  the  same  apparent, 
optic  angle  in  air,  about  118°.  However,  in  diopside,  the  value 
/?,  which  is  1.678,  is  decidedly  higher  than  in  orthoclase,  where 
/?  =  1.524.  Thus,  in  the  former  the  apparent  optic  angle  corre- 
sponds to  a  much  smaller  real  angle,  61 1/2°,  while  the  real  angle 
of  orthoclase  is  71°. 

Only  the  apparent  optic  angle  can  be  measured  in  a  microscope. 
The  distance  between  the  vertices  of  the  hyperbolas  is  used  for 
this  determination,  which  is  made,  if  possible,  in  monochromatic 
light.  With  a  micrometer  scale  in  the  ocular  the  law  of  Mallard 
can  be  used;  sin  E  =  DK,  where  D  is  the  number  of  divisions  on  the 
scale  and  K  is  the  constant  determined,  once  for  all  time,  for  one 
of  the  objectives.  It  can  be  determined  best  with  a  plate  cut 
perpendicular  to  the  acute  bisectrix  with  a  known  apparent  optic 
angle. 

In  using  the  simplest  methods  of  observation  in  convergent  light  there  is 
generally  no  need  of  a  micrometer  scale  in  the  ocular.  Such  a  scale  can, 
however,  be  placed  in  the  objective,  so  that  it  can  be  seen  sharply  at  the  same 
time  as  the  interference  figure,  or  it  can  be  etched  on  the  front  lens  of  the 
objective.  This  has  a  disadvantage,  however,  that  such  an  objective  can- 
not be  used  so  much  for  ordinary  observations  and,  further,  that  the  image 
produced  by  this  method  is  extremely  small,  and  the  error  in  the  measure- 
ment is  therefore  very  large. 

The  measurement  of  the  apparent  optic  angle  can  be  best  made  with  the 
microscope  by  using  a  Bertrand  or  Klein  lens,  each  of  which  produces  an 
enlarged  interference  figure.  With  a  Bertrand  Jens,  which  transforms  the 
ocular  into  a  microscope,  measurement  can  be  made  by  means  of  an  ocular 
micrometer,  i.e.,  by  means  of  a  scale  placed  in  the  focus  of  the  ocular  itself. 
With  a  Klein  lens,  on  the  other  hand,  the  scale  can  be  shifted  in  the  holder 
of  the  lens  which  is  placed  over  the  ocular.  When  the  proper  adjustment 
of  the  image  and  scale  has  been  attained,  there  should  be  no  relative  dis- 
placement when  the  eye  is  moved  back  and  forth  over  the  lens.  Otherwise 
there  is  considerable  error  resulting  from  the  parallax. 

If,  instead  of  a  fixed  micrometer,  the  cross  hairs  in  the  ocular  are  so 
arranged  as  to  be  moved  by  means  of  a  micrometer  screw,  their  intersection 
8 


114 


PETROGRAPHIC  METHODS 


can  be  made  to  coincide  in  turn  with  the  vertices  of 
the  hyperbolas.  The  amount  of  the  displacement  can 
be  read  off  on  the  drum  of  the  screw.  This  method 
gives  results  that  are  a  little  more  accurate  than 
with  a  fixed  micrometer.  Finally  the  interference 
figure  can  be  transferred  to  coordinate  paper  by 
means  of  a  sketching  device.  Now  and  then  this 
method  has  many  advantages. 

The  sine  of  the  optic  angle  is  obtained  from  the 
distance  between  the  hyperbolas,  and  the  simplest 
way  to  make  this  determination  is  to  use  a  sine  rule, 
for  example,  Schwarzmann's  optic  angle  scale,  Fig.  143. 
The  two  parts  a  and  b  slide  on  each  other.  The  upper 
part  a  gives  the  number  of  divisions  in  the  ocular  mi- 
crometer and  the  lower  part  b  the  size  of  the  corres- 
ponding optic  angle  when  it  has  been  adjusted  for  the 
ocular  used.  In  Fig.  143  it  is  adjusted  by  measuring 
the  optic  angle  of  aragonite  on  a  fixed  micrometer  in 
sodium  light.  There  were  5.9  divisions  on  the  scale 
corresponding  to  an  apparent  optic  angle  of  30°  15'. 
30°  15'  on  the  lower  scale  is  placed  exactly  under  5.9 
of  the  upper,  which  constitutes  adjustment  of  the 
scale  for  that  objective.  If  a  section  of  topaz  is 
studied,  17.6  scale  divisions  will  be  noted.  The  divi- 
sion on  the  lower  scale  corresponding  to  that  is  99°, 
which  is  the  apparent  optic  angle  for  topaz. 

The  scale  has  another  value,  viz.,  that  the  real  optic 
angle  can  be  deduced  from  the  apparent  without  any 
calculation  if  the  value  of  /?  is  known.  The  value  of 
£  is  marked  in  the  upper  scale,  for  example  1.61  for 
topaz,  and  x  the  distance  from  division  1  to  the  new 
division  is  measured  with  a  pair  of  dividers.  This 
distance,  when  measured  backward  from  the  apparent 
optic  angle,  gives  the  value  of  the  real  angle,  which  in 
this  case  is  about  5.6°. 

The  optic  angle  can  also  be  measured  in  those 
sections  which  are  not  exactly  orientated.  This  is 
very  important  for  petrographic  investigation  because 
perfectly  orientated  cross  sections  are  rare  in  slides. 
The  trace  of  the  optic  axes  is  best  transferred  to  a 
projection  from  which  the  calculation  of  the  optic 
angle  can  be  made,  if  the  intermediate  index  of  re- 
fraction is  known.  These  measurements  are  not  of 
very  great  importance  in  the  microscopic  study  of 
rocks,  because  in  most  of  the  isomorphous  groups, 
which  are  the  most  widespread  constituents  of  rocks, 
the  optic  angle  varies  within  wide  limits  and  no  rela- 
tionship appears  to  exist  between  the  optic  angle  and 
the  chemical  composition. 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  115 

Character  of  the  Double  Refraction  of  Biaxial  Minerals. — The 

determination  of  the  character  of  double  refraction  of  biaxial 
minerals  is  important  only  when  it  it  positively  known  whether 
the  determination  was  made  on  the  acute  or  obtuse  bisectrix. 
It  frequently  happens  in  rock-forming  minerals  that  the  optic 
angle  is  not  far  from  90°,  as  in  plagioclase,  olivine,  epidote,  and 
then  the  distinction  is  very  difficult.  The  optic  axes  do  not  then 
emerge  from  a  section  perpendicular  to  the  acute  bisectrix  any 
more  than  they  do  in  a  section  perpendicular  to  the  obtuse  bisec- 
trix; thus  this  characteristic  of  the  mineral  must  be  abandoned 
because  a  positive  acute  bisectrix  corresponds  to  a  negative 
obtuse  bisectrix,  and  vice  versa. 


FIGS.  144-145. — Determination  of  the  Character  of  the  Double  Refraction  in  a  Section 
Oblique  to  an  Optic  Axis. 

Sections  that  show  the  emergence  of  an  optic  axis  in  the  field 
of  vision  of  the  microscope  are  very  important.  If  the  optic 
angle  is  not  too  near  90°  the  dark  bar  produced  by  a  slightly 
oblique  axis  is  distinctly  curved  with  the  convex  side  toward  the 
acute  and  the  concave  side  toward  the  obtuse  bisectrix.  When 
the  plane  of  the  optic  axes  is  placed  at  45°  to  the  nicols  and  violet 
I  is  inserted,  the  bars  appear  violet  but  the  convex  and  concave 
sides  of  the  curves  are  colored  differently  as  indicated  in  Figs. 
144  and  145.  The  axial  plane  in  each  case  is  parallel  to  the 
direction  of  the  greatest  elasticity  in  the  test  plate,  i.e.,  the  bars 
are  perpendicular  to  it.  Hence  in  a  negative  crystal  the  convex 
side  is  yellow  and  the  concave  blue,  Fig.  144,  and  this  is  reversed 
in  Fig.  145.  In  that  portion  of  the  image  toward  the  bisectrix  a, 
the  ray  vibrating  in  the  axial  plane  is  propagated  with  the  velocity 
c.  In  this  case  the  axial  plane  lies  parallel  to  a  in  the  compensator 
and  hence  subtraction  of  the  double  refraction  takes  place  on  this 
side  of  the  axis.  The  interference  color  sinks  from  violet  in  the 
bar  to  yellow. 


116  PETROGRAPHIC  METHODS 

With  a  section  about  perpendicular  to  a  bisectrix,  the  change 
produced  by  a  quarter  undulation  plate  inserted  at  45°  to  the 
nicols  is  observed,  when  the  axial  plane  is  parallel  to  the  vibration 
direction  of  one  of  the  nicols.  The  black  cross  disappears  and 
the  curves  in  each  pair  of  opposite  quadrants  are  either  moved 
closer  together  or  farther  apart.  Thinning  of  the  rings  takes 
place  in  those  quadrants  cut  by  the  direction  of  greatest  elas- 
ticity in  the  test  plate  when  the  crystal  is  positive,  and  vice 
versa  if  it  is  negative.  This  test  can  only  be  made  with  safety 
in  well  orientated  sections  and  is  much  less  used  than  the  method 
which  depends  upon  compensation  of  the  double  refraction. 

When  the  interference  figure  is  placed  in  the  45°  position  with 
respect  to  the  nicols,  the  middle  of  the  field  shows  an  interference 
color  which  corresponds  to  the  retardation  of  the  rays  propagated 
in  the  direction  of  the  bisectrix  in  question.  One  of  these  rays 
vibrates  parallel  to  the  axial  plane  and  is  propagated  with  the 
velocity  belonging  to  the  ray  whose  vibration  direction  is  the 
other  bisectrix.  The  second  ray  vibrates  parallel  to  the  optic 
normal  b  and  has  the  corresponding  intermediate  velocity.  If, 
for  example,  the  section  is  perpendicular  to  the  acute  bisec- 
trix of  a  negative  crystal,  that  ray  which  vibrates  in  the  axial 
plane  is  parallel  to  the  obtuse  bisectrix,  which  in  this  case  is  the 
direction  of  least  elasticity  for  the  crystal  and  is  therefore  smaller 
than  the  value  of  the  optic  normal  perpendicular  to  it.  A 
compensator  is  then  inserted  so  that  its  direction  of  greatest 
elasticity  is  parallel  to  the  axial  plane  in  the  crystal.  Thus, 
equivalent  directions  are  crossed  and  the  impression  is  obtained 
that  the  thickness  of  the  crystal  has  been  diminished.  The 
interference  colors  are  lowered  and  the  rings  spread  out.  A 
biaxial  crystal  is  negative  if  the  double^  refraction  is  lowered, 
when  the  direction  of  greatest  elasticity  in  the  compensator  is 
parallel  to  the  axial  plane  in  a  section  perpendicular  to  the  acute 
bisectrix,  and  positive  when  it  is  increased.  The  reverse  of  this 
is  true  for  a  section  perpendicular  to  the  obtuse  bisectrix. 

Beginners  often  experience  difficulty  in  making  this  determination  with 
feebly  double  refracting  substances,  which  show  no  color  in  the  field,  or 
only  gray  of  the  first  order.  This  is  especially  true  for  those  groups  of  rock- 
forming  minerals  on  which  observations  in  convergent  polarized  light  are 
most  frequently  made,  as  the  piagioclases.  With  sections  perpendicular  to 
the  negative  bisectrix  of  such  crystals,  the  original  gray  changes  to  yellow 
when  violet  I,  which  is  most  frequently  used  for  these  determinations,  is 
inserted.  Apparently  the  interference  color  is  raised  instead  of  lowered  but 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  117 


the  gray  in  the  slide  has  actually  been  subtracted  from  the  red  of  the  com- 
pensator, and  yellow  remains.  This  fact  can  be  proved  by  rotating  the 
compensator,  for  the  field  becomes  blue,  i.e.,  it  acquires  the  next  higher 
color  after  violet  I,  while  the  yellow  is  the  next  one  lower  and  signifies 
subtraction  of  the  interference  colors. 

It  is  often  interesting  to  determine  the  position  of  the  axial  plane  with 
respect  to  the  edges  of  a  crystal,  this  being  sometimes  a  valuable  characteris- 
tic of  the  mineral.  The  interference  figure  is  placed  in  the  45°  position. 
The  line  joining  the  vertices  of  the  hyperbolas  in  the  interference  figure,  or 


FIG.  146. — Biaxial  Interference  Figures  in  Sections  Oblique  to  the  Acute 
Bisectrix. 

the  line  perpendicular  to  one  'arm  of  the  curve  is  the  direction  of  the  plane 
of  the  optic  axes.  It  is  unimportant  which  method  of  observation  in  con- 
vergent light  is  used,  because  the  rotation  of  the  image  is  of  no  consequence 
in  determining  the  direction  of  the  plane. 

This  rotation  of  the  image  is  considered  when  the  bisectrix  is  inclined 
toward  tho  vertical,  and  it  must  be  determined  in  which  direction  the  devia- 
tion takes  place.  It  must  always  be  remembered  that  in  observing  without 
the  ocular,  the  interference  figure  is  reversed  with  respect  to  the  image  of 
the  object,  while  in  observing  with  the  ocular  the  two  images  are  placed 
parallel. 

The  phenomena  produced  by  a  biaxial  crystal  cut  obliquely  to  the  prin- 
cipal vibration  directions  are  shown  in  Fig.  146.  It  represents  the  behavior 
of  three  differently  orientated  sections  of  topaz  upon  a  rotation  through  90°. 
Unlike  the  behavior  of  uniaxial  crystals,  Fig.  118,  p.  101,  it  can  be  distinctly 
seen  that  the  dark  bar  rotates  about  the  point  of  emergence  of  the  axis 
as  a  center,  because  in  this  case  its  direction  is  not  dependent  upon  the 
position  of  the  principal  sections  of  the  nicols  alone,  but  upon  the  relation  of 
them  to  the  principal  vibration  directions  of  the  plate. 


118  PETROGRAPHIC  METHODS 

Besides  sections  perpendicular  to  the  acute  bisectrix  and  those  perpen- 
dicular to  the  optic  axes,  which  have  been  amply  described  above,  sections 
perpendicular  to  the  obtuse  bisectrix  and  those  parallel  to  the  plane  of  the 
optic  axes  are  of  special  interest.  If  the  real  optic  angle  of  a  certain  sub- 
stance is  nearly  90°,  it  is  difficult  to  distinguish  between  acute  and  obtuse 
bisectrices  especially  in  minerals  with  low  indices  of  refraction.  If  one  does 
not  wish  to  make  accurate  measurements,  or  if  they  cannot  be  made,  because 
there  is  no  objective  with  a  sufficient  aperture,  very  useful  results  can  be  ob- 
tained in  parallel  polarized  light.  With  two  well  orientated  sections  of  equal 
thickness,  perpendicular  respectively  to  each  of  the  bisectrices,  the  one 
perpendicular  to  the  obtuse  bisectrix  gives  the  higher  interference  color.  As 
the  size  of  the  acute  optic  angle  decreases,  and  the  obtuse  becomes  larger,  the 
difference  of  the  interference  colors  of  the  two  sections  naturally  increases. 
In  convergent  light  the  lemniscates  in  the  interference  figure,  perpendicular 
to  the  obtuse  bisectrix,  are  wider,  and  the  dark  broadened  bar  cuts  them 
only  at  the  extreme  edge  01  Jie  field.  The  change  is  the  same  as  if  the  obser- 
vation were  made  with  objectives  with  constantly  decreasing  magnification. 
The  reverse  order  of  the  images  in  Fig.  114,  p.  98,  gives  the  best  idea  of  this 
gradual  change.  When  the  apparent  obtuse  optic  angle  becomes  very 
large,  there  is  only  an  indication  of  the  symmetrical  distribution  of  color  in 
the  different  quadrants,  when  observed  in  white  light.  The  dark  bars  corre- 
sponding to  the  hyperbolas  first  appear  in  the  field,  when  the  plane  of  the 
optic  axes  is  almost  perfectly  parallel  to  the  vibration  directions  of  the  nicols. 
They  are  then  very  broad  and  not  sharply  defined  so  that  it  gives  the  im- 
pression that  the  plate  is  alternately  light  and  dark  throughout  its  whole 
extent. 

The  phenomena  described  above  can  be  observed  in  plates  parallel  to  the 
plane  of  the  optic  axes.  They  cannot  be  distinguished  in  white  light  from 
a  plate  perpendicular  to  the  bisector  of  a  large  obtuse  optic  angle  or  those 
parallel  to  the  optic  axis  of  a  uniaxial  mineral.  They  can  be  distinguished 
sometimes  in  monochromatic  light  because  in  a  section  perpendicular  to  the 
obtuse  bisectrix  black  curves  occur  corresponding  to  the  colored  ones  in 
white  light.  These  curves  are  portions  of  lemniscates  which  curve  toward  the 
point  of  emergence  of  the  optic  axes  near  the  edge  of  the  field,  Fig.  132,  p.  109. 
Analogous  curves  in  a  section  parallel  to  the  axial  plane  of  a  biaxial  crystal 
or  to  the  optic  axis  of  a  uniaxial  crystal  are  hyperbolas,  which  do  not  show 
such  bending.  If  this  reaction  does  not  possess  the  necessary  sharpness  for 
positive  differentiation,  it  is  still  helpful,  under  certain  conditions,  to  observe 
the  interference  figure  in  monochromatic  light  because  with  it  the  orienta- 
tion of  the  section  can  be  determined  much  better.  The  determination 
of  the  direction  of  extinction  in  a  section  of  a  monoclinic  crystal  should  never 
be  undertaken  before  it  is  proved  in  convergent  polarized  light  that  the  sec- 
tion is  really  parallel  to  the  clinopinacoid  of  the  crystal,  which  is  always 
perpendicular  to  one  of  the  three  principal  vibration  directions.  It  always 
gives  an  interference  figure  in  monochromatic  light,  which  is  symmetrical 
with  respect  to  two  directions  perpendicular  to  each  other. 

The  direction  of  the  acute  bisectrix  can  be  recognized  distinctly  by  the 
distribution  of  colors  in  the  interference  figure  in  the  45°  position  of  a  section 
parallel  to  the  axial  plane  in  the  same  manner  as  was  described  on  page  102  for 


OBSERVATIONS,  CONVERGENT  POLARIZED  LIGHT  119 

sections  parallel  to  the  optic  axis  of  a  uniaxial  mineral.  It  is  necessary,  how- 
ever, that  the  real  optic  angle  does  not  approximate  90°  too  closely.  In  the 
quadrants  through  which  the  acute  bisectrix  passes  there  is  a  lowering  of  the 
color  or  when  the  optic  angle  is  quite  large  a  slight  raising  of  the  color.  In 
the  other  two  quadrants  the  color  is  raised  and  in  case  the  optic  angle  is 
large,  the  increase  is  much  greater  than  in  the  other  two  quadrants.  The 
following  figures  give  the  values  for  gypsum,  which  is  optically  positive  and 
has  an  optic  angle  of  59°  corresponding  to  the  table  on  page  103.  The  values 
under  I  are  for  the  planes  inclined  to  the  acute  bisectrix  and  those  under  II 
for  those  inclined  to  the  obtuse  bisectrix. 

I  II 


u 

P 

d 

w 

d 

w 

0° 

1.0000 

0.0098 

0.0098 

0.0098 

0.0098 

20° 

1.0652 

0.0089 

0.0095 

0.0096 

0.0100 

40° 

1.3054 

0.0066 

0.0086 

0.0089 

0.0116 

60° 

2  .  0000 

0.0040 

0.0080 

0.0082 

0..0164 

The  rays  in  plane  I,  inclined  about  60°  to  the  normal  to  the  plate,  show 
not  quite  half  the  retardation  of  the  corresponding  directions  in  plane  II. 


CHAPTER  VI 


Twins  and  Optical  Anomalies 

Twins. — Many  crystals  do  not  consist  of  a  single  homogeneous  individual 
but  of  two  or  more  individuals  intergrown  in  a  regular  manner.     They  are 
called  twins,  fourlings,  etc.     The  crystallographic  laws  according  to  which 
such  intergrowth  takes  place  can  be  determined  in  simple  cases  by  careful 
microscopic  observation,  but  with  more  complex  inter- 
growths  the  results  of  such  observations  must  be  used 
with   great   care,    if  they   have    not  been  verified  by 
accurate  goniometric  measurements. 

Twinned  intergrowths  of  cubic  crystals  can  only  be 
determined  under  the  microscope  by  their  outline. 
Reentrant  angles  are  a  clue  to  the  presence  of  twins, 
Fig.  147.  Optical  recognition  of  twins  is  not  possible 

FIG.    147. Cubic     m  uniaxial  minerals,  if  the  various  sections  are  parallel 

Twin  with  Reentrant  to  the  principal  axis.  The  vibration  directions  and 
Angle.  Magnetite.  flte  velocities  of  the  extraordinary  rays  are  the  same 
for  both  individuals  in  every  section  through  such  a 
crystal.  In  convergent  polarized  light  the  behavior  of  the  various  indi- 
viduals is  the  same  in  this  case.  Quartz  is  a  characteristic  example.  It 
forms  intergrowths  and  twins  having  parallel  optic  axes,  which  cannot  be 
determined  microscopically. 

When  the  principal  axes  of  the  twinned 
individuals  are  inclined  to  each  other  it  will 
be  observed  in  every  section  oblique  or  per- 
pendicular to  the  twinning  plane,  that  the 
parts  do  not  extinguish  simultaneously. 
Thus  in  Fig.  148,  when  one  part  of  the  twin 
is  dark,  the  other  part,  sharply  separated 
from  it  by  the  twinning  plane,  is  light,  and 
vice  versa.  The  angle,  which  the  extinction 
directions  in  the  two  individuals  form  with 
each  other  must  not  always  be  taken  as  a 
basis  for  the  calculation  of  the  twinning  law, 
because  the  true  inclination  of  the  two 
principal  axes  can  only  be  determined  when 
the  slide  is  perpendicular  to  the  twinning 
plane. 

Twins  in  the  orthorhombic  system  are  generally  more  easily  determined. 
A  difference  in  color  in  various  parts  of  strongly  pleochroic  substances  is 
indicative  of  the  presence  of  twins,  even  though  the  principal  axes  are 
parallel.  Such  twins  are  difficult  to  recognize,  however,  in  case  the  pleo- 

120 


FIG.  148.— Uniaxial  Twin  with 
Inclined  Axes.     Rutile. 


TWINS  AND  OPTICAL  ANOMALIES 


121 


chroism  is  not  distinct.  This  is  also  the  case  when  the  axial  plane  is  per- 
pendicular to  the  twinning  plane  and  the  optic  angle  is  very  large,  or  the 
bisectrix  of  a  very  small  optic  angle  lies  in  the  twinning  plane.  In  both 
these  cases  the  extinction  directions  coincide  and  the  interference  colors  are 
quite  similar  in  the  sections  in  which  the  twinning  can  usually  be  recognized 
most  easily.  In  such  cases  the  orientation  of  the  different  parts  can  be 
established  with  convergent  polarized  light.  If,  on  the  other  hand,  the 
crystallographic  axes  of  the  twinned  individuals  are  oblique  to  each  other, 
the  intergrowth  can  be  recognized  by  different  extinction  directions.  The 
two  individuals  appear  equally  light  when  the  inclination  of  each  to  the 
nicols  is  the  same,  Fig.  149. 

Most  of  the  twins  in  the  monoclinic  system  are  those  in  which  the  6  axes 
of  the  two  individuals  are  parallel.  If  the  crystals  are  developed  prismatic 
to  this  axis,  the  zone  of  the  6  axis  will  be  the  one  most  frequently  studied. 
What  was  said  for  orthorhombic  crystals  with  parallel  principal  axes  applies 
also  for  sections  of  monoclinic  inclined  at  a  large  angle  to  the  6  axis.  Such 
twins  cannot  be  positively  determined  in  all  cases  by  optical  means.  This- 
is  seen  in  the  extremely  common  twins  of  epidote,  which  are  but  rarely 
recognizable.  But  the  different  positions  of  the  vibration  directions  in  the 
twins  can  be  easily  determined  in  sections  perpendicular  or  oblique  to  the 
b  axis,  as  shown  in  the  example  of  diopside,  Fig.  150. 

Twins  of  triclinic  crystals  can  be  recognized  in  all  sections  because  the 
vibration  directions  do  not  generally  lie  parallel. 


FIG.  149. — Penetration 
Twin  with  Inclined  Axes. 
Staurolite. 


FIG.  150. — Monoclinic 
Twin.     Diopside. 


FIG.  151. — Penetration 
Trilling.     Cordierite. 


It  is  sometimes  noted  that  not  only  two,  but  three  or  even  more  indi- 
viduals are  intergrown  in  a  regular  manner.  Intergrowths  of  three  indi- 
viduals are  called  trillings.  Characteristic  penetration  trillings  of  certain 
orthorhombic  and  monoclinic  crystals  are  noteworthy.  They  possess  a 
prism  angle  of  approximately  120°,  and  unite  to  form 'an  apparently  hex- 
agonal crystal,  as  shown  in  Fig.  151. 

Fourlings  generally  assume  the  form  of  lamellar  intergrowths.  Twinning 
lamination  is  particularly  common  in  monoclinic  and  triclinic  substances 
but  is  not  lacking  in  orthorhombic  (olivine),  hexagonal  (calcite),  tetragonal 
(rutile),  and  in  cubic  crystals  (fluorite).  In  the  latter  it  cannot  be  observed 
optically.  Crystals,  twinned  in  this  manner,  apparently  consist  of  two  or 


122 


PETROGRAPHIC  METHODS 


more  individuals  which  penetrate  each  other  in  a  lamellar  manner,  e.g.,  pla- 
gioclase,  Fig.  152,  so  that  one  system  of  lamellae  belongs  to  one  individual 
and  the  other  to  a  second,  etc.  It  sometimes  happens  that  one  system  of 
twinning  lamellae,  produced  by  twinning  according  to  a  certain  law,  is 
crossed  by  another  system  belonging  to  a  second  law.  The  phenomenon 
may  assume  very  fine  development  and  is  called  lattice  lamination  or  cross 
hatching.  Fig.  153  shows  cross  hatching  in  albite  produced  by  twinning 
according  to  the  albite  and  pericline  laws. 

This  complicated  structure  of  crystals  consisting  of  numerous  alternating 
twinning  lamellae  is  very  widespread  especially  in  the  larger  polymorphous 
groups,  e.g.,  the  epidote  and  feldspar  groups.  Modifications  of  lower  sym- 
metry approach  those  of  higher  symmetry  with  respect  to  external  form 
by  such  repeated  twinning.  The  approximation  is  the  greater  the  finer  the 
individuals,  until,  finally,  a  minuteness  of  the  single  components  may  be 


FIG.  152. — Twinning  Lamination. 


FIG.  153. — Lattice  Lamination. 


attained,  such  that  they  cannot  be  differentiated  under  the  microscope. 
Then  the  optical  properties  agree  with  those  of  the  modifications  with  higher 
symmetry.  This  gradual  transition  from  lower  to  higher  symmetry  gave 
rise  to  the  theory  of  Mallard,  that  numerous  crystals  belonging  to  groups 
with  higher  symmetry  are  the  result  of  such  twinning  of  individuals  with 
lower  symmetry. 

It  has  often  been  observed  that  substances  which  crystallize  in  one  crystal 
system  at  a  certain  temperature,  are  resolved  into  a  pile  of  complicated 
twins  of  individuals  with  lower  symmetry  when  the  temperature  is  changed. 
This  was  shown  long  ago  for  leucite  and  boracite,  which,  at  the  moment  of 
their  formation  at  higher  temperatures,  crystallize  cubic,  but  upon  cooling 
this  modification  is  not  stable  and  passes  over  into  an  apparently  confused 
complex  of  double  refracting  lamellae,  Fig.  154.  The  cubic  condition  of 
equilibrium  can  be  restored  by  heating  the  crystal  up  to  a  certain  tempera- 
ture, while  cooling  is  followed  again  by  the  breaking  up  into  the  double 
refracting  modification. 

Many  methods  have  been  used  to  determine  the  monoclinic  and  triclinic 
crystals  composed  of  twinning  lamellae,  which  occur  so  frequently  in  rocks. 
The  sections  of  such  crystals  are  orientated  in  various  ways  and  the  methods 
employed  for  their  determination  are  based  in  part  upon  the  mutual 
relations  of  the  twinned  individuals.  The  simplest  method  is  the  determi- 
nation of  the  extinction  angle  in  a  series  of  sections  in  which  the  two  indi- 


TWINS  AND  OPTICAL  ANOMALIES  123 

viduals  extinguish  symmetrically  with  respect  to  the  twinning  plane,  i.e., 
they  are  perpendicular  to  the  twinning  plane.  Another  method  is  the 
determination  of  the  position  of  equal  luminosity,  which  very  thin  plates 
of  feebly,  double  refracting  crystals  show.  Thicker  plates,  or  those  with 
higher  double  refraction,  can  only  be  used  in  monochromatic  light.  A 
twin,  in  which  the  vibration  directions  are  oblique  to  each  other  in  the  two 
parts,  cannot  be  recognized  as  such  in  all  positions  between  crossed  nicols. 
On  rotating  the  crystal  horizontally  through  360°,  there  are  eight  positions 
in  which  the  two  halves  appear  equally  light  and  the  twinning  plane  entirely 
disappears.  Four  of  these  are  crossed  with  respect  to  the  other  four. 
These  two  groups  of  positions  of  four  each  can  be  distinguished  from  each 
other  in  that  when  the  lamella  partially  overlap  like  wedges,  these  over- 
lapping parts  in  one  group  are  just  as  light  as  the  rest  of  the  crystal,  but  in 
the  other  group  they  are  darker.  This  method  of  determining  the  position 
of  equal  luminosity  was  formerly  successfully  employed  in  the  determina- 
tion of  the  plagioclases,  but  it  did  not  find  any  general  application  and  has 
been  replaced  by  modern  methods  of  determination. 


FIG.  154. — Twinning  Lamination  FIG.  155. — Segments  in 

in  Leucite.  Garnet. 

Optical  Anomalies. — Optical  anomalies  are  quite  often  observed  in  micro- 
scopic studies.  Variations  in  the  optic  angle  of  biaxial  crystals,  the  slight 
opening  of  the  dark  cross  in  uniaxial  crystals  upon  rotating  in  convergent 
polarized  light,  and  the  uneven  illumination  of  cubic  crystals  between  crossed 
nicols  are  very  common  occurrences.  A  very  faintly  illuminated  halo  cut 
by  a  dark  cross,  Brewster's  cross,  often  appears  around  inclusions  in  glass 
and  cubic  crystals  in  parallel  polarized  light. 

Crystals  with  an  external  cubic  form  may  be  sometimes  observed,  which 
do  not  show  normal  optical  behavior  either  in  their  entirety  or  in  regularly 
bounded  portions.  They  appear  double  refracting.  The  phenomenon  in 
leucite  and  its  decomposition  into  a  complex  of  twins  has  been  mentioned 
above..  Strictly  speaking,  this  phenomenon  cannot  be  classed  with  anom- 
alies as  generally  considered,  but  it  is  a  genuine  and  characteristic  para- 
morph.  A  characteristic  type  of  optical  anomalies  for  cubic  crystals  may  be 
observed  when  a  second  substance  is  present  in  such  minute  quantities  that 
it  cannot  be  detected  chemically.  The  two  substances  may  or  may  not 
be  isomorphous.  Thus  a  crystal  of  potassium  alum  may  contain  slight 
amounts  of  ammonium  alum  and  retain  its  outward  cubical  symmetry  but 
internally  it  is  composed  of  a  number  of  double  refracting  pyramids. 

It  can  generally  be  observed  that  the  structure  of  such  a  crystal  is  most 


124  PETROGRAPHIC  METHODS 

intimately  related  to  its  external  form.  It  is  composed  of  as  many  pyramids 
as  there  are  faces,  and  each  face  of  the  crystal  is  the  base  of  a  pyramid,  the 
apexes  of  which  unite  in  the  center  of  the  crystal,  Fig.  155.  If  the  substance 
is  without  definite  crystal  form,  its  internal  structure  cannot,  naturally,  be 
influenced  by  it.  The  optical  properties,  which  are  based  upon  the  internal 
structure  of  the  crystal,  are  disturbed  before  the  development  of  the  form  is 
completed.  Such  a  division  of  the  field  into  segments  is  observed  quite 
frequently  in  cubic  crystals  and  not  infrequently  also  in  uniaxial  crystals. 
They  are,  however,  rarer  the  lower  the  symmetry  of  the  crystal,  because 
anomalous  crystals  always  correspond  in  their  optical  behavior  to  groups 
with  lower  symmetry.  It  is  noteworthy  that  certain  compounds  are  very 
susceptible  to  such  optical  influences  so  that  they  are  rarely  found  in  a 
normal  condition, -while  in  others  optical  anomalies  are  not  known  at  all. 

The  phenomena  grouped  together  as  optical  anomalies  of  isotropic  bodies 
are  of  three  kinds.  1.  An  uneven  illumination  between  crossed  nicols  with 
a  Brewster's  cross  in  the  neighborhood  of  inclusions.  This  is  undoubtedly 
produced  by  tension  and  is  found  in  amorphous  as  well  as  in  cubic  sub- 
stances. 2.  A  division  of  the  field  corresponding  to  the  external  form.  This 
is  generally  produced  by  foreign  substances  between  the  molecules  and  is 
likewise  a  tension  phenomenon.  3.  The  occurrence  of  twin  laminations  or 
cross  hatching.  These  can  generally  be  referred  to  dimorphism  of  the  sub- 
stance in  question,  and  paramorphism  resulting  from  it. 


Appendix 
Accessory  Apparatus 

A  number  of  more  or  less  complicated  devices  are  quite  desir- 
able in  some  cases  as  accessories  to  the  microscope.  They  are 
of  great  importance  in  certain  special  investigations,  and  are 
mentioned  here  for  the  sake  of  completeness.  It  is  not  intended 
to  give  a  long  list  of  them  or  a  detailed  description  of  each  but 
to  give  some  idea  only  of  their  general  applicability.  There  are 
three  principal  groups  of  such  accessory  apparatuses  which  are 
of  great  importance. 

1.  Rotation  apparatus. 

2.  Heating  apparatus. 

3.  Projection  and  reproduction  apparatus. 

The  more  important  types  of  these  will  be  briefly  described. 

i.  Rotation  Apparatus 

Under  this  head  are  included  all  those  simple  or  complex 
attributes  to  the  microscope  by  which  the  object  under  investiga- 
tion can  be  rotated  about  one  or  more  axes  other  than  the  axis  of 
the  stage.  The  oldest  of  these  were  used  exclusively  for  gonio- 
metric  investigation.  They  were  called  microscope  goniometers 
but  they  are  scarcely  of  any  importance  at  the  present  time. 
There  is  a  whole  series  of  adjustments  from  the  simplest  types  to 
the  universal  stage,  which  are  used  principally  for  investigating 
the  optical  properties  of  an  object  in  various  directions. 

These  devices  depend  chiefly  upon  two  different  principles. 
One  tends  to  eliminate  refraction  and  total  reflection  of  the  rays 
coming  from  the  oblique  surfaces  of  the  preparation.  This  is 
accomplished  by  placing  the  crystal  or  the  microscopic  prepara- 
tion between  two  plano-convex  lenses  so  that  at  all  points  both 
surfaces  of  the  apparatus  are  perpendicular  to  the  rays  of  light, 
which  are  sent  through  the  preparation,  and  no  deviation  of  the 
central  rays,  at  least,  can  take  place.  The  other  makes  it 
possible  to  observe  an  object  immersed  in  a  liquid  of  very  similar 
index  of  refraction.  The  liquid  is  in  the  form  of  a  horizontal  or 

125 


126  PETROGRAPHIC  METHODS 

a  vertical  layer  bounded  by  plane  surfaces,  and  the  crystal  can 
be  rotated  in  it  in  any  manner.  The  first  method  is  not  very 
valuable  in  the  study  of  thin  sections.  The  other  is  used  more 
frequently  with  isolated  crystals  but  can  also  be  employed  very 
profitably  with  thin  sections. 

Rotation  Apparatus  for  Observations  between  Two  Plano-convex 
Lenses. — The  simplest  of  these  accessories  was  suggested  by 
Schroeder  van  der  Kolk  and  consits  only  of  a  single  pair  of  lenses. 
One  lens  with  a  diameter  of  25  mm.  is  placed  in  the  opening  of 
the  stage  and  can  be  rotated  in  all  directions.  The  other  lens 
with  a  diameter  of  about  8  mm.  is  placed  on  the  center  of  the  slide. 
Both  lenses  must  be  so  made  that  the  preparation  lies  as  nearly  as 
possible  in  the  focus  of  the  system,  i.e.,  the  lower  lens  must  differ 
from  the  form  of  a  hemisphere  by  the  thickness  of  the  object 
glass  and  the  upper  one  by  the  thickness  of  the  cover-glass.  The 
object  glass  of  the  preparation,  which  should  be  circular  and 
have  a  smaller  diameter  than  the  lens  itself,  is  laid  upon  the  larger 
hemisphere,  contact  being  made  by  a  drop  of  glycerine.  The 
particle  to  be  investigated  is  then  centered  accurately  when  the 
preparation  is  as  nearly  horizontal  as  possible.  The  second 
lens  is  fastened  on  the  cover-glass  of  the  preparation  by  a  small 
drop  of  glycerine  also.  This  lens  is  moved  about  until  the  object 
to  be  observed,  which  appears  twice  as  large  as  before,  is  exactly 
in  the  middle  of  the  field  of  vision.  The  two  principal  vibration 
directions  of  the  object  are  determined  and  then  the  lower  lens 
is  rotated  about  one  of  the  directions  as  an  axis,  permitting 
observations  to  be  made  in  parallel  polarized  light  continuously. 
An  object  holder,  Fig.  33,  page  19,  can  be  used  to  eliminate  many 
of  the  accidents  which  may  result  by  the  rotation  by  hand.  It 
rests  on  the  preparation  like  a  spring  and  can  be  rotated  about  a 
horizontal  axis.  The  amount  of  rotation  can  be  read  off  a  small 
circular  scale. 

The  principal  vibration  direction  of  the  object  under  investigation  is 
placed  parallel  to  the  axis  of  rotation  of  the  apparatus  and  then  the  rotation 
can  be  carried  out  accurately  and  measured.  With  a  prismatic  crystal, 
which  shows  parallel  extinction  and  gives  no  characteristic  interference 
figure  in  convergent  light,  the  rotation  is  made  with  the  long  axis  of  the 
crystal  as  the  axis  of  rotation  until  it  lies  parallel  to  one  of  the  vibration 
directions  of  the  nicols.  Two  cases  are  possible.  Either  the  crystal  remains 
constantly  extinguished  during  the  rotation,  or  more  and  more  illumination 
appears  and,  after  a  definite  rotation  of  the  accessory  apparatus,  the 
crystal  must  be  rotated  on  a  vertical  axis  through  a  definite  arc  to  produce 


APPENDIX  127 

darkness  again.  In  the  latter  case  the  crystal  is  monoclinic.  If  it  remains 
constantly  dark  it  is  rotated  on  a  second  axis  perpendicular  to  the  first 
horizontal  axis  and  the  observations  repeated  in  the  same  order.  Now, 
if  it  remains  dark  during  the  whole  rotation  the  crystal  is  tetragonal,  hex- 
agonal or  orthorhombic,  but  if  it  gets  gradually  lighter  during  the  second 
rotation  it  is  monoclinic  with  a  prismatic  development  parallel  to  the  6  axis. 

If  there  is  parallel  extinction  in  all  cases  the  long  direction  of  the  crystal 
is  used  as  the  axis  of  rotation  and  it  is  placed  at  45°  to  the  nicols  so  that  the 
crystal  shows  its  most  brilliant  interference  color.  If,  when  the  crystal  is 
rotated,  there  is  no  change  in  the  interference  color,  it  is  uniaxial.  If  there 
is  a  change,  it  is  orthorhombic.  It  is  thus  obvious  that  this  apparatus  is  an 
extremely  important  aid  in  the  investigation  of  crystals.  All  the  changes 
which  take  place  in  convergent  polarized  light  can  be  followed  by  lengthen- 
ing the  tube  slightly  without  otherwise  changing  the  adjustments. 

The  apparatus  would  be  much  more  useful  if  it  were  constructed  of  a 
series  of  large  lenses  with  a  hemispherical  cup  cut  in  the  center  of  the  plane 
surface.  Large  crystals,  or  precious  stones,  could  be  immersed  in  this  cup 


FIG.  156. — Fedorow  Universal  Stage. 

in  a  liquid  with  the  same  index  of  refraction  and  the  various  directions 
studied  in  parallel  and  convergent  light.  This  gives  excellent  results  in  the 
study  of  precious  stones. 

There  are  a  large  number  of  other  apparatuses  of  this  kind  based  upon 
the  same  principle  but  only  the  large  model  of  the  Fedorow  universal  stage 
will  be  described  in  detail.  Fig.  156  shows  this  stage  with  three  axes  of 
rotation.  It  consists  of  a  stand  llll  which  can  be  placed  on  the  stage  of  a 
microscope.  The  stage  in  the  stand  can  be  rotated  about  a  horizontal  axis 
by  means  of  the  screw  k.  The  circle  T  with  its  vernier  n  shows  the  amount 
of  rotation.  The  axis  can  be  fixed  by  means  of  the  screw  /.  The  vernier 
nl  is  firmly  fixed  on  this  axis  and  the  stage  K  is  rotated  on  an  axis  perpen- 
dicular to  the  first.  The  stage  K  bears  a  third  axis  TLd  which  can  be  rotated 
about  some  axis  lying  in  the  plane  of  this  stage.  Finally  the  glass  stage  S 
carrying  the  preparation  can  be  rotated  in  its  own  plane. 

The  preparation  is  placed  on  the  stage,  contact  being  made  by  glycerine. 
Then  a  plano-convex  lens  a  is  placed  under  the  stage  S  and  another  over  the 
preparation  concentrically.  Here  also  glycerine  serves  to  insure  a  good 
contact.  The  efficiency  of  the  apparatus  can  be  increased  for  the  investiga- 


128 


PETROGRAPHIC  METHODS 


tion  of  crystals  with  higher  indices  of  refraction,  by  using  a  more  strongly 
refracting  glass  for  the  stage,  lenses  and  holders  of  the  object.  The  liquid 
used  for  the  contact  must  also  have  a  higher  index  of  refraction  than  in  the 
former  case.  Although  this  apparatus  has  proved  its  usefulness  for  certain 
investigations,  for  example,  in  the  study  of  the  feldspars,  the  results  obtained 
are  not  commensurate  with  its  complicated  construction.  The  necessity 
of  having  to  use  special  kinds  of  glass  with  this  apparatus  naturally  prevents 
its  being  used  extensively. 

Rotation  Apparatus  for  Investigation  in  Liquids. — This  method 
was  first  proposed  by  C.  Klein  and  used  for  studying  isolated 

crystals  or  fragments,  but  was  later 
also  employed  in  the  investigation 
of  thin  sections.  The  simplest  ap- 
paratus consists  of  a  low  glass  dish 
the  bottom  of  which  is  a  plane 
parallel  glass  plate.  On  one  side 
there  is  a  tapering  neck  in  which  a 
glass  stopper  can  be  rotated.  The 
inner  end  of  this  stopper  is  pro- 
vided with  a  crystal  holder  and  the 
outer  end  with  a  scale,  Fig.  157. 
The  dish  is  filled  with  a  liquid  hav- 
ing as  nearly  as  possible  the  same 
index  of  refraction  as  the  crystal  to 
be  studied.  The  crystal  is  placed 

in  the  holder  exactly  parallel  to  the  zone  under  consideration. 
The  extinction  direction  on  the  various  faces  in  the  zone  can  be 
determined  with  this  apparatus.  The  real  optic  angle  can  also 
be  measured  directly  if  the  index  of  refraction  of  the  liquid 
corresponds  exactly  with  that  of  the  crystal. 

Special  objectives  with  a  wide  focal  angle  and  not  too  small 
a  field  as  well  as  the  appropriate  condensers  generally  accom- 
pany this  rotation  apparatus,  so  that  observations  in  convergent 
polarized  light  can  be  made  although  object  and  objective  are 
separated  considerably. 

Klein's  larger  universal  rotation  apparatus  for  crystals  can  be 
used  more  extensively.  The  crystal  can  be  quite  accurately  ad- 
justed and  centered  on  its  holder  and  the  measurements  made 
with  considerable  precision.  For  using  this  apparatus  the  micro- 
scope is  placed  in  a  horizontal  position  and  a  small  rectangular 
glass  dish  placed  in  the  path  of  the  rays  of  light.  This  dish  con- 
tains the  liquid  in  which  the  crystal  is  immersed,  and  can  be 


FIG.  157. — Simple  Rotation  Appa- 
ratus by  R.  Fuesz. 


APPENDIX  129 

rotated  without  striking  the  sides.  The  plane  parallel  walls  are 
placed  as  nearly  perpendicular  as  possible  to  the  axis  of  the 
microscope. 

A  small  goniometer  can  also  be  combined  with  the  stage  of  a  microscope 
in  a  similar  manner.  It  however  has  the  disadvantage  that  either  the  focal 
length  of  the  objective  must  be  much  larger  on  account  of  the  elevation  of 
the  adjusting  and  centering  apparatus  on  the  goniometer,  or  the 'goniometer 
must  be  placed  entirely  outside  of  the  axis  of  the  microscope  and  the  crystal 
mounted  on  an  especially  long  holder.  In  the  latter  case,  the  lever  arm  is  so 
long  that  a  very  slight  movement  of  the  adjusting  screws  produces  so  large 
a  displacement  of  the  crystal  that  such  a  device  has  not  proved  practical. 

The  apparatus  shown  in  Fig.  158  is  constructed. for  investiga- 
tion of  thin  sections  according  to  this  method.  The  dish  B, 
which  holds  the  liquid,  is  closed  at  a  by  a  plane  parallel  glass 


FIG.  158. — Universal  Rotation  Apparatus  for  Slides  by  C.  Klein. 

plate.  S  is  the  stage,  the  middle  part  of  which  consists  of  glass. 
The  object  is  fixed  on  it  by  means  of  the  clamps  e  and  e1.  The 
screw  k  is  used  to  rotate  the  stage  S  in  its  own  plane,  the  motion 
being  transmitted  by  a  cog  wheel.  With  T  it  can  be  moved  on  a 
second  axis  perpendicular  to  the  first.  The  instrument  is  made 
for  various  sizes  of  object  glasses.  Ordinary  slides  can  be  studied 
with  it  but  the  cover-glass  and  any  adhering  Canada  balsam  must 
first  be  removed. 

2.  Heating  Apparatus 

Since  the  optical  properties  of  crystals  are  dependent  upon  the 
temperature  and  in  some  substances  change  considerably  with 
comparatively  slight  variations  in  temperature,  a  number  of 
devices  have  been  constructed  for  making  microscopic  observa- 
tions at  high  and  constant  temperatures.  Such  an  apparatus  is 
9 


130 


PETROGRAPHIC  METHODS 


also  used  to  observe  crystallization  in  its  incipient  stages  at 
higher  temperatures.  For  this  reason  a  microscope  equipped 
with  such  an  apparatus,  Fig.  159,  is  also  called  a  crystallizing 
microscope,  and  can  be  used  especially  for  physical-chemical  in- 
vestigations. Very  high  temperatures  can  be  obtained  with  it 
without  injuring  the  lenses  because  the  lenses  of  the  objective 
are  continually  water-cooled. 


FIG.  159. — Chemical  Microscope  by  Voigt  &  Hochgesang. 

In  a  special,  heating  microscope  recently  constructed  by 
Doelter  an  electric  current  is  used  to  produce  the  heat.  With 
regulating  devices,  high  and  very  constant  temperatures  can  be 
obtained.  With  the  microscope  shown  in  Fig.  160,  which  is  also 


APPENDIX 


131 


arranged  for  photography  at  high  temperatures,  temperatures  of 
over  1000°  with  a  magnification  of  132  can  be  obtained  by  using  a 
small  furnace.  These  temperatures  can  be  controlled  accurately 
in  intervals  of  5°  and  can  be  held  constant  for  a  long  time.  With 
larger  furnaces  higher  temperatures  up  to  1600°  can  be  obtained 
but  with  a  smaller  magnification,  up  to  88.  Observations  in  po- 
larized light  are  possible  up  to  about  1200°,  when  the  object  itself 


KS 


FIG.  160. — Electric    Heating    Microscope   with   Photographic    Camera    by    Doelter. 

Reichert  in  Vienna). 


(C. 


is  too  strongly  luminous.  The  furnaces  used  are  well  insulated 
and  are  closed  top  and  bottom  with  a  quartz-glass  plate.  The 
lenses  of  the  objective  are  not  cemented  and  are  water-cooled. 
In  Fig.  160  this  heating  microscope  is  combined  with  a  photo- 
graphic camera. 

A  very  useful  heating  apparatus  for  smaller  tests  is  shown  in  Fig.  161  at 
about  1/3  natural  size .  It  is  covered  with  asbestos  and  placed  upon  f our[glass 
legs  to  insulate  it  from  the  stage  and  the  optical  apparatus  in  the  stage. 
The  heating  box  A'  is  closed  at  6  and  the  opposite  under-side  by  ajplane 


132  PETROGRAPHIC  METHODS 

parallel  glass  plate,  while  the  heated  air  passes  through  the  flue  A.  The 
preparation  is  placed  on  an  object  holder  located  in  the  hole  b  and  heated  by 
means  of  the  gas  burner  gg' '.  To  cool  it  off  rapidly,  cold  air  can  be  intro- 
duced through  the  tube  r.  The  thermometer  on  the  flue  A  can  be  read  to 
450°.  It  rests  on  the  object  like  a  fork  and  thus  the  temperature  can  be 
determined  with  great  accuracy. 


P 
FIG.  161. — Projection  Apparatus  by  W.  &  H.  Seibert. 

3.  Projection  and  Reproduction  Apparatus 

There  are  a  large  number  of  devices  made  to  obtain  and  repro- 
duce microscopic  images  without  any  subjective  influence.  In 
so  far  as  such  a  reproduction  can  be  made  by  microphotography, 
the  method  deserves  preference  on  account  of  its  absolute  objec- 
tivity. There  are,  however,  a  whole  series  of  phenomena  which 
a  photographic  plate  does  not  reproduce  with  sufficient  distinct- 
ness, especially  when  the  image  possesses  some  comparatively 
unimportant  feature  the  importance  of  which  should,  however, 
be  emphasized  for  certain  purposes. 

Microphotographic  and  Projection  Apparatus. — Any  micro- 
scope can  be  adapted  for  projection  and  demonstration  purposes 
with  microscopic  preparations,  by  using  a  source  of  light  with  a 
system  of  lenses,  as  shown  in  Fig.  162.  An  electric  arc  is  the 
best  source  of  light  but  if  electricity  is  not  available,  a  lime  or 
zircon  light  or  a  Welsbach  burner  can  be  used.  The  intensity  of 
light  diminishes  in  the  order  named.  The  real  image  produced 
by  the  objective  is  best  for  projection,  at  least  with  the  com- 
paratively low  magnifications  generally  used  in  a  polarizing 
microscope.  An  image,  a  yard  in  diameter,  can  be  projected  by 
using  an  arc  light  and  a  medium  magnification.  It  may  be 
thrown  either  on  an  opaque  white  screen  or  on  a  white  transparent 


APPENDIX 


133 


screen  and  viewed  from  the  other  side,  but  it  must  be  exactly 
focused  by  means  of  the  micrometer  screw  of  the  microscope. 
The  large  amount  of  heating  produced  by  the  system  of  lenses 
LjL/2  must  be  avoided  by  inserting  a  cooling  bath  with  circulating 


FIG.  162. — Projection  Apparatus  by  W.  &  H.  Seibert. 

water  K  in  the  path  of  the  rays.  Even  then  great  care  is  neces- 
sary on  account  of  the  sensitiveness  of  the  Canada  balsam  in  the 
nicols.  As  the  nicols  must  be  left  so  long  in  the  path  of  the  rays, 

even  though  cooled,  it  is  advisable  to 
use  a  microscope  equipped  with  an  ad- 
justment for  throwing  out  the  polarizer. 
The  larger  and  more  complicated 
microphotographic  apparatuses  can  be 
left  entirely  out  of  consideration  for 
reproduction  of  the  phenomena  seen  in 
a  polarizing  microscope,  because  their 
magnification  is  comparatively  unim- 
portant to  the  petrographer.  Perfectly 
good  results  can  be  obtained  with  any 
camera  after  a  little  practice,  provided 
it  is  properly  set  up  with  the  micro- 
scope, and  there  is  a  strong,  well-cen- 
tered source  of  light.  The  real  image 
of  the  object  produced  by  a  microphotographic  objective, 
corrected  for  the  chemically  active  light  rays,  is  reproduced 
on  the  plate.  A  projection  ocular  is  used  only  for  stronger 
magnifications.  The  simple  apparatus  represented  in  Fig. 


FIG.  163. — Photographic  Camera 

by  R.  Fuesz. 
(1/5  Natural  Size). 


134 


PETROGRAPHIC  METHODS 


163,  which  can  be  set  on  the  microscope,  is  very  good  for  low 
magnifications. 

Photomicrographs  taken  in  diffused  daylight  often  lack  in 
sharpness,  which  is  the  result  of  long  exposure  and  not  very 
careful  manipulation.  Various  sources  of  artificial  light  are 
much  to  be  preferred  on  account  of  the  greater  constancy  of  the 
luminosity  of  these  sources.  An  arc  or  lime  light  is  the  best. 
Recourse  to  color  filters,  which  play  so  great  a  r6le  in  organic 
microphotography,  is  had  here,  only  in  special  cases  particularly 
for  photography  at  very  high  temperatures. 

Photographing  with  convergent  polarized  light  is  compara- 
tively simple.  The  real  image  from  a  Bertrand  lens  without  the 


FIG.  164. — Drawing  Apparatus  by  Abbe. 

ocular  is  focused  directly  on  a  photographic  plate.  The  Bertrand 
lenses  for  ordinary  investigations  generally  have  too  small  an 
aperture  to  project  the  image  beyond  the  tube  of  the  microscope, 
and  hence  special  lenses  with  larger  focal  lengths  are  needed  for 
photography  and  projection  purposes.  It  may  also  be  remarked 
that  colored  photographs  of  interference  figures  have  been  suc- 
cessfully made  by  the  use  of  Lumiere  plates. 

Drawing  Apparatus. — There  is  a  large  number  of  drawing 
devices  depending  upon  the  principle  of  the  camera  lucida. 
The  image  observed  in  the  microscope  is  thrown  upon  a  sheet  of 
drawing  paper,  where  it  appears  uniformly  distinct  over  the 
whole  field  and  can  be  traced  with  a  sharp  pointed  pencil.  Two 
of  these  constructions  have  been  found  to  be  especially  useful. 
The  first  of  these  is  the  Abbe  drawing  apparatus,  the  second  the 


APPENDIX 


135 


FIG.  165. — Nachet  Drawing 
Apparatus. 


Nachet.     The  former  is  to  be  preferred  because  there  is  no  loss 
of  light  even  when  the  strongest  objectives  are  used. 

The  Abbe  apparatus,  Fig.  164,  consists  of  two  prisms  RR'  cemented  to- 
gether, forming  a  cube.  They  are  silvered  on  the  plane  of  contact  with  the 
exception  of  a  small  spot  in  the  center.  The  eye  receives  the  image  o  from 
the  ocular  through  this  opening,  while  a  mirror  placed  at  a  distance  of  70  mm. 
reflects  an  image  of  the  drawing  paper  on  the  silvered  surface  from  which  it 
is  reflected  to  the  eye.  The  difference  in 
intensity  of  luminosity  of  the  object  and 
the  drawing  surface  can  be  regulated  by 
inserting  plates  of  smoked  glass  with  differ- 
ent depths  of  color  in  the  path  of  the  rays 
from  the  latter. 

The  Nachet  drawing  apparatus,  Fig.  165, 
consists  of  a  prism  with  a  rhombic  cross 
section  abed  on  the  front  of  which  a  small 
prism  efg  is  cemented  over  the  ocular.  The 
rays  o'  coming  from  the  object  strike  the 
face  ef  perpendicularly  and  pass  to  the  eye 

unrefracted.     The  rays  from  the  drawing  pencil  p  suffer  reflection  at  cb 
and  ad,  and  then  pass  out  of  the  apparatus  the  same  as  the  others. 

It  is  obvious  that  an  interference  figure  can  also  be  drawn  with  this 
apparatus.  It  is  very  useful  for  measurements  under  certain  conditions, 
especially  when  the  interference  figure  has  been  traced  on  cross-section 
paper. 

Summary  of  Methods 

When  all  the  methods  that  may  be  employed  for  determining 
crystals  with  a  polarizing  microscope  are  assembled,  the  following 
are  noted: 

1.  Observation  of  the  index  of  refraction. 

2.  Determination  of  crystal  form  and  cleavage. 

3.  Observation  of  inclusions. 

4.  Determination  of  color  and  pleochroism,  including  obser- 
vations in  reflected  light. 

5.  Recognition  of  double  refraction. 

6.  Determination  of  the  position  of  vibration  directions. 

7.  Measurement  of  the  double  refraction. 

8.  Determination  of  the  optical  character  of  the  principal 
zone. 

9.  Distinction  between  uniaxial  and  biaxial  crystals. 

10.  Determination  of  the  optical  character. 

11.  Determination  of  the  position  of  the  optic  plane. 


136  PETROGRAPHIC  METHODS 

12.  Measurement  of  the   optic   angle   and  determination  of 
the  dispersion. 

Except  for  the  sake  of  practice  for  beginners  it  is  not  necessary 
to  adhere  strictly  to  the  order  given. 

In  work  with  the  polarizing  microscope  with  a  definite  object 
in  view,  the  crystal  system  is  first  determined  and  then  the  vari- 
ous properties  necessary  for  the  determination  of  the  substance 
are  observed. 

The  determination  can  only  be  made  between  crossed  nicols 
and  in  many  instances,  especially  with  biaxial  crystals,  observa- 
tions in  parallel  light  must  be  combined  with  those  in  convergent 
light. 

The  following  scheme  is  given  for  this  determination:  ' 

1.  All  individuals  of  a  substance  remain  dark  between  crossed 
nicols  through  a  rotation  of  360°:  Cubic  crystals. 

2.  Most  individuals  become  alternately  light  and  dark  in  a 
rotation  through  360°  between  crossed  nicols:  Not  cubic 
crystals. 

2a.  The  vibration  directions  always  lie  parallel  or  sym- 
metrical to  the  outlines  of  the  crystal:  Hexagonal, 
tetragonal,  or  orthorhombic  crystals. 

26.  The  vibration  directions  lie  partly  parallel  or  sym- 
metrical and  partly  oblique  to  the  outlines:  Mono- 
clinic  crystals. 

2c.  The  vibration  directions  always  lie  oblique  to  the 
outlines:  Triclinic  crystals. 

2a1$  A  uniaxial  interference  figure  is  obtained  in  con- 
vergent polarized  light:  Hexagonal  and  tetragonal 
crystals. 

2a2.  A  biaxial  interference  figure  is  obtained  in  convergent 
polarized  light:  Orthorhombic  crystals. 

Crystals  of  the  hexagonal  and  tetragonal  systems  can  be  dis- 
tinguished from  each  other  only  by  crystal  form  and  cleavage. 

This  schematic  arrangement  can  be  modified  in  its  detail,  and  there  is  a' 
considerable  difference  between  the  investigation  of  isolated  crystalline 
powders  or  cleavage  fragments  and  determinations  in  thin  sections.  In  the 
former  case  the  individuals  are  frequently  all,  or  nearly  all,  orientated  in 
the  same  manner  on  account  of  crystallographic  development  or  perfectness 
of  cleavage  and  lie  on  one  certain  crystal  face.  In  thin  sections,  on  the  other 
hand,  accurate  crystallographic  orientation  occurs  only  in  exceptional  cases. 
The  cross  sections  are  orientated  in  various  ways  rendering  the  determina- 


APPENDIX  137 

tion  of  the  crystal  system  easier.     These  two  cases  will  be  considered 
separately. 

When  the  observations  are  made  on  a  powder  and  none  of  the  individuals 
affect  polarized  light,  the  conclusion  cannot  be  drawn  that  the  substance  is 
cubic.  Whether  or  not  it  is  uniaxial  and  all  the  individuals  accidentally  lie 
on  the  basal  pinacoid  can  be  determined  by  observations  in  convergent  light. 
In  a  thin  section  determinations  in  convergent  light  are  rarely  necessary. 

If  there  are  individuals  of  a  substance  in  the  powder  or  in  a  thin  section, 
which  give  different  interference  colors  when  the  thickness  is  the  same, 
recourse  is  taken  at  once  to  observations  in  convergent  light.  Whether  the 
substance  is  uniaxial  or  biaxial  can  be  best  determined  on  those  individuals 
which  show  the  lowest  interference  color,  because  these  are  most  inclined  to 
an  optic  axis  which  will  appear  within  the  field  of  vision.  It  will  be  very 
difficult  for  a  beginner  to  find  all  those  individuals  of  one  substance,  especially 
when  the  crystals  are  colorless.  The  interference  colors  may  be  different 
and  this  may  be  accompanied  by  a  difference  in  the  whole  habit  in  various 
directions.  The  appearance  of  colorless  mica  in  thin  section  may  be  taken 
as  an  example.  All  sections  transverse  to  the  basal  pinacoid  show  lath- 
shaped  crystals,  sharp  cleavage  cracks,  and  high  interference  colors.  Sec- 
tions of  the  same  mineral  parallel  to  the  base  have  very  weak  or  no  double 
refraction  on  account  of  the  small  optic  angle.  There  is  also  no  trace  of 
cleavage  or  elongation.  The  mineral  appears  this  way  also  when  powdered. 
The  brilliant  colors  observed  in  the  interference  figure  are  proof,  however, 
that  the  mineral  has  strong  double  refraction. 

If  it  has  been  found  that  the  substance  in  a  powder  is  biaxial,  an  attempt 
should  be  made  to  obtain  an  interference  figure  as  nearly  symmetrical  as 
possible.  If  the  interference  figure,  obtained  from  an  individual  with 
symmetrical  extinction,  is  symmetrical  to  two  planes,  the  crystal  is  ortho- 
rhombic.  If  it  is  symmetrical  to  one  plane  only  it  may  be  monoclinic. 
The  latter  is  certainly  true  if  the  fragment  shows  oblique  extinction  in 
parallel  polarized  light.  If  the  crystals  have  a  prismatic  development  or  a 
prismatic  cleavage  they  can  be  determined  as  orthorhombic  by  the  parallel 
extinction  of  the  individuals  oblique  to  the  plane  of  the  optic  axes.  Dis- 
tinction between  monoclinic  and  triclinic  individuals  is  very  difficult 
because  individuals  with  oblique  extinction  are  also  oblique  to  the  axes,  as 
is  generally  the  case  in  triclinic  crystals. 

The  distinction  of  uniaxial  from  biaxial  crystals  in  powders  -is  often  very 
difficult  when  the  crystals  are  elongated  parallel  to  the  principal  axis  or  to 
the  acute  bisectrix  or  when  they  have  good  cleavages  parallel  to  these  faces. 
All  individuals  then  lie  approximately  parallel  to  the  corresponding  princi- 
pal optical  directions.  An  interference  figure  parallel  to  the  optic  axis  of  a 
uniaxial  crystal  and  that  in  a  section  parallel  to  the  plane  of  the  optic  axes 
or  perpendicular  to  the  obtuse  bisectrix  are  of  such  a  character  that  even  an 
experienced  observer  may  often  draw  erroneous  conclusions  from  them. 
When  many  crystals  of  approximately  the  same  size  are  observed,  it  is 
noted  that  all  needles  of  uniaxial  crystals  show  the  same  interference  colors 
for  the  same  thickness  and,  when  embedded  in  a  mobile  liquid  and  rolled  by 
the  movement  of  it,  the  interference  color  does  not  change.  In  biaxial 
crystals,  however,  there  will  be  a  difference  when  the  thickness  is  the  same, 


138 


PETROGRAPHIC  METHODS 


depending  upon  Whether  the  observation  is  made  parallel  to  the  optic  nor- 
mal or  the  obtuse  bisectrix,  and  this  change  can  be  verified  when  the 
crystal  is  rolled  by  the  movement  of  the  liquid. 

The  beginner  usually  experiences  difficulty  in  determining  the  crystal 
system,  partly  on  account  of  the  optical  anomalies  and  partly  on  account  of 
the  great  similarity  which  the  systems  of  lower  symmetry  show  to  those  of 
higher  symmetry  in  their  optical  behavior.  Optical  anomalies  in  cubic 
crystals  are  often  of  such  magnitude  that  they  show  quite  brilliant  inter- 
ference colors  and  in  basal  sections  of  uniaxial  minerals  the  double  refrac- 
tion may  be  so  low  that  no  definite  reaction  can  be  obtained  either  in  con- 
vergent or  parallel  polarized  light  and  the  mineral  may  be  considered  cubic. 
Such  is  the  case  in  a  cross  section  of  apatite. 

Uniaxial  minerals  frequently  show  a  transition  to  biaxial  by  the  opening 
of  the  dark  cross  of  the  interference  figure  when  rotated,  for  example,  quartz, 
and  vesuvianite,  while  biaxial  crystals  approach  uniaxial  by  a  small  optic 


£  =  Bluish -green 
A      ^#=  Green 


^Yellowish- 
green 


FIQ.  166. 


Fio.  167. 


Optical  Sketch  of  a  Crystal. 


angle,  thus,  phlogopite.  Monoclinic  and  triclinic  crystals  assume  the 
properties  of  orthorhombic  when  their  oblique  extinction  cannot  be  meas- 
ured, for  example,  muscovite  and  epidote,  and  oblique  sections  of  ortho- 
rhombic  minerals  with  a  decided  deviation  from  symmetrical  extinction  can 
sometimes  be  observed.  The  determination  of  the  crystal  system  in  such 
cases  can  be  made  accurately  only  by  a  skilled  observer. 

Further  investigation  of  minerals,  after  the  crystal  system  has  been  de- 
termined, is  best  carried  out  systemmatically  according  to  the  scheme  given 
on  page  135,  using  the  material  in  the  corresponding  chapters  as  a  basis.  All 
the  optical  properties  of  the  crystal  investigated  can  be  shown  in  a  sketch 
as  represented  in  Figs.  166  and  167. 

These  methods  of  crystal  drawing  have  proved  very  practical  and  give 
a  clear  idea  of  the  results  of  the  microscopic  study.  After  the  form,  cleavage, 
inclusions,  etc.,  have  been  drawn  as  true  to  nature  as  possible  with  the  aid 
of  a  drawing  apparatus,  the  vibration  directions  are  determined  and  indi- 
cated on  the  drawing  by  arrows.  The  determination  of  the  relative  values 


APPENDIX  139 

of  two  rays  vibrating  in  a  cross  section,  combined  with  the  investigations 
in  convergent  light,  gives  the  directions  of  the  axes  of  greatest,  medium,  and 
least  elasticity  which  are  indicated  in  the  figure  by  c,  B  and  c.  The  index 
of  refraction  in  any  direction  is  indicated  in  the  following  way:  if  it  corre- 
sponds with  that  of  Canada  balsam  the  corresponding  arrow  is  drawn  very 
lightly;  if  it  is  still  lower  the  arrow  is  dotted;  if  it  is  higher  the  arrow  is  drawn 
heavier  and  is  the  heavier  the  greater  the  difference  between  the  index  of 
refraction  for  the  corresponding  vibration  direction  and  that  of  Canada 
balsam.  If  pleochroism  is  present,  it  is  determined  and  the  arrow  for  the 
corresponding  direction  is  labeled  accordingly.  The  amount  of  the  double 
refraction  estimated  from  the  interference  color  is  indicated  by  arcs  connect- 
ing the  arrows  for  the  vibration  directions.  Here,  again,  very  low  double 
refraction  is  indicated  by  dotted  lines,  while  the  heavier  lines  indicate 
stronger  double  refraction.  When  the  double  refraction  is  very  strong,  the 
lines  are  doubled.  Investigation  in  convergent  light  reveals  whether  the 
substance  is  uniaxial  or  biaxial.  If  the  former  is  the  case  and  the  axis 
emerges  perpendicularly,  a  small  circle  with  a  black  cross  is  drawn  in  the 
center  of  the  sketch.  If  the  plane  of  the  drawing  is  parallel  to  the  axis  the 
same  signal  is  made  outside  of  the  sketch.  If  the  section  is  oblique  the 
interference  figure  is  sketched  on  the  edge  of  the  crystal  about  as  it  appears 
in  the  microscope.  If  the  crystal  is  biaxial,  the  true  position  of  the  optic 
plane  is  indicated  and  the  symmetry  of  the  emergence  of  the  axes  as  well  as 
the  approximate  size  of  the  optic  angle  is  shown  in  the  sketch.  There  are, 
naturally,  cases  in  which  a  sketch  of  this  character  does  not  give  the  entire 
perspective  of  the  optical  properties,  but  in  a  large  number  of  cases  detailed 
descriptions  are  unnecessary,  and  a  conception  of  all  the  optical  properties  of 
a  crystal  can  be  had  at  a  glance,  as  far  as  they  can  be  determined  by  the 
qualitative  methods  of  microscopic  technic.  Fig.  166  is  based  upon  crys- 
tals of  hydrated  basic  copper  sulphate,  and  Fig.  167  upon  an  artificially 
prepared  manganese  salt  corresponding  to  vivianite, 


PART  II 

ROCK-FORMING  MINERALS 


CHAPTER  VII 
Preparation  of  Material 

Investigation  of  Rock  Powder. — Rock  powders  were  used  for  the  earliest 
studies  of  rocks  under  the  microscope.  The  powder  on  the  slide  was  covered 
with  water  (n  =  1.333)  to  reduce  the  total  reflection  as  much  as  possible. 
Later,  cedar  oil  (n  =  1.52)  or  Canada  balsam  (n  =  1.545)  was  used.  This 
very  simple  method  can  still  be  used  to  get  very  good  results  in  determining 
rapidly  the  approximate  properties  of  a  rock  or  a  mineral.  There  may  be 
too  little  of  some  of  the  constituents  to  determine  them  in  the  usual  way  and 
then  recourse  is  taken  to  this  method,  in  which  case  the  residues  of  chemical 
or  mechanical  separations  are  used.  There  is  a  large  number  of  minerals 
that  are  difficult  to  determine  in  a  slide,  but  which  show  characteristic 
features  in  powdered  form.  This  is  particularly  true  for  minerals  that 
cleave  easily.  Cleavage  plates  may  be  studied  in  convergent  light,  and 
minerals  with  similar  characters  can  be  easily  distinguished.  Scapolite,  the 
different  members  of  the  mica  group  and  similar  .minerals,  topaz,  cyanite,  and 
prehnite  are  some  which  in  many  instances  can  be  positively  determined 
in  cleavage  plates  only. 

Table  19,  I  will  aid  in  these  determinations.  The  best  method  of  pre- 
paring the  material  is  to  crush  a  small  grain  of  the  mineral  between  two 
object  glasses  by  pressing  between  the  fingers.  The  larger  pieces  resulting 
from  this  treatment  are  placed  on  another  object  glass  and  covered  with 
eugenol,  which  has  an  index  of  refraction  the  same  as  Canada  balsam 
(n  =  1.545).  Finally  a  cover-glass  is  placed  on  the  preparation. 

This  method  is  especially  applicable  in  the  study  of  porous  materials  such 
as  sand,  and  in  the  investigation  of  soil  of  any  sort.  The  index  of  refraction 
of  the  grains  may  be  determined  accurately  according  to  the  Schroeder  van 
der  Kolk  method  (see  Part  I,  page  38).  This  is  of  great  importance  in 
the  determination  of  the  feldspars  which  are  otherwise  difficult  to  recognize 
in  a  powder.  (See  feldspar  group,  method  No.  7.) 

Various  grains  of  a  mineral  in  a  powder  may  have  different  thickness. 
This  may  make  them  appear  to  have  different  optical  properties,  which 
gives  rise  to  much  confusion  in  all  investigations  on  powders,  especially  for 
a  beginner.  Slides,  however,  can  be  prepared  from  such  material  in  the 
following  manner:  the  powder  is  mixed  with  a  thick  paste  of  zinc  oxide 
and  potassium  water  glass  and  the  mass  is  left  to  harden  in  a  short  glass 
tube  of  large  diameter.  A  slide  is  made  of  the  mixture,  thus  prepared,  by 
the  method  of  making  rock  sections,  to  be  described  below. 

It  will  thus  be  seen  that  the  microscopic  investigation  of  rock  powders  is 
often  quite  important  and,  since  these  methods  frequently  lead  to  conclusions 
quite  different  from  those  obtained  in  studying  a  thin  section,  they  should 
be  strongly  emphasized  in  petrographical  studies. 

143 


144  PETROGRAPHIC  METHODS 

In  some  cases,  which  are,  however,  quite  infrequent,  certain  clues  are  ob- 
tained by  observing  the  crystallographic  form  of  the  rock  constituents. 
Great  care  must  be  taken  in  the  preparation  of  such  material  that  the  grains 
are  not  broken.  The  isolated  material  is  examined  either  without  any 
liquid  or  immersed  in  one  such  as  water,  which  has  an  index  of  refraction 
widely  different  from  that  of  most  of  the  rock-forming  minerals. 

The  rock  is  coarsely  crushed  in  a  mortar  for  mechanical  and  chemical 
separations.  Then  by  pounding  and  hammering,  but  not  by  rubbing,  a 
powder  is  prepared  in  which  the  grains  are  as  nearly  of  equal  size  as  possible. 
Most  of  these  grains  consist  of  but  a  single  mineral.  Grains  of  various  sizes 
are  isolated  by  means  of  a  series  of  sieves  of  from  25  to  250  meshes  per  square 
inch.  The  fine  dust,  which  is  formed  in  large  amounts  in  all  these  opera- 
tions, is  thus  removed.  This  is  quite  necessary,  at  least  for  the  mechanical 
methods  of  separation,  because  the  dust  would  remain  suspended  for  a  long 
time  in  the  rather  viscous  liquids  used  in  these  operations.  The  dust  is 
only  used  where  there  are  microlitic  constituents  in  the  rock  to  be  studied. 
On  the  other  hand,  a  fine  dust-like  condition  is  especially  favorable  for 
chemical  methods  of  separation  and,  in  this  case,  it  is  the  coarser  grains  that 
must  be  removed. 

It  is  evident  that  the  material  to  be  studied  and  prepared  by  these 
various  methods  should  be  as  fresh  as  possible  and  must  not  be  taken  from 
a  weathered  surface  or  otherwise  altered  portion  of  the  rock.  The  material 
should  be  carefully  studied  first  under  the  microscope  before  any  further 
investigations  or  attempts  at  separation  are  made. 

Preparation  of  Thin  Sections. — Thin  sections  are  the  most  im- 
portant preparations  for  the  study  of  rocks;  not  only  because  a 
large  number  of  variously  orientated  mineral  sections  are  shown 
side  by  side,  but  also  because  the  texture  of  a  rock,  which  is  so 
important  in  petrography,  can  be  studied. 

The  preparation  of  thin  sections,  especially  of  the  harder  rocks, 
was  formerly  an  operation  requiring  unlimited  time.  They  were 
ground  by  hand  on  an  iron  plate  covered  with  wet  emery  powder. 
This  method  is  still  used  for  polishing  small  rock  chips.  The 
operation  has  been  gre'atly  simplified  by  modern  cutting  and 
polishing  machines  but,  in  spite  of  this,  it  is  nevertheless  a 
tedious  task  requiring  much  practice  and  skill.  It  is  advisable 
to  have  such  work  done  by  a  specialist,  as  for  instance  Voigt 
und  Hochgesang  in  Goettingen,  Germany,  or  W.  H.  Tomlinson, 
in  Swarthmore,  Pa.  Nevertheless,  every  petrographer  should  be 
able  to  prepare  usable  sections.  This  may  be  necessary  in  some 
cases  where  a  knowledge  of  a  rock  is  desired  at  once.  For  this 
reason,  the  principal  hand  machines  will  be  briefly  described 
here. 

A  small  cutting  and  polishing  machine  driven  by  hand  or,  if 


PREPARATION  OF  MATERIAL 


145 


the  necessary  power  plant  is  at  hand,  one  equipped  to  run  by 
power  is  an  indispensable  part  of  the  equipment  of  a  modern 
petrographical  laboratory.  Fig.  168  illustrates  such  a  machine 
designed  by  Voigt  and  Hochgesang.  Thin  plates  of  quite  hard 
rocks  can  be  cut  in  a  comparatively  short  time  by  means  of  a 
rapidly  rotating  metallic  disk  set  with  bort,  emery,  or  carborun- 
dum powder.  The  plates  can  then  be  ground  down  on  the 
polishing  disk. 


FIG.  168. — Cutting  and  Polishing  Machine. 


The  polishing  plate  consists  of  a  slightly  convex  iron  disk,  a  lead  plate 
being  used  for  the  coarser  manipulations.  It  can  be  made  to  rotate  rapidly. 
At  first  coarse  powder,  emery  or  carborundum,  is  placed  on  it  with  water, 
while  fine  powder  is  used  for  the  finer  polishing  operations.  The  plate  to  be 
polished  is  pressed  by  hand,  or  by  a  mechanical  device,  as  evenly  as  possible 
on  the  rotating  disk  until  a  plane  surface  is  obtained.  Then  it  is  rubbed 
with  fine  wet  powder  and,  finally,  with  dust  entirely  free  from  grains  until 
the  surface  is  polished.  Thin  sections  are  not  given  a  high  polish  except 
10 


146  PETROGRAPHIC  METHODS 

in  special  cases,  e.g.,  when  they  are  to  be  used  with  a  Wallerant  total 
reflectometer  (see  Part  I,  p.  40). 

When  one  surface  has  been  ground  as  carefully  as  possible,  it  is  firmly 
cemented  to  a  small  glass  plate.  Canada  balsam,  which  is  used  as  a  cement, 
is  placed  on  the  glass  and  evaporated  on  a  water  bath  until  it  is  hard  but 
not  brittle.  Then  it  is  heated  carefully  until  it  becomes  fluid  and  the 
ground  fragment  is  warmed  and  carefully  pressed  into  it.  The  fragment 
should  be  moved  back  and  forth  slightly  to  remove  the  air  bubbles  from  the 
balsam.  When  no  more  bubbles  can  be  seen  through  the  glass  the  fragment 
is  held  firmly  for  a  moment  until  the  balsam  has  completely  cooled.  Then 
the  same  grinding  process  is  carried  out  on  the  other  side  of  the  fragment, 
but  naturally  great  care  must  be  taken  that  the  abrasive  agent  does  not 
tear  a  portion  of  the  fragment  away.  The  final  polishing  is  accomplished 
under  special  precautions  using  only  a  very  small  quantity  of  the  finest 
grinding  material.  It  is  tested  from  time  to  time  under  the  microscope  to 
see  whether  the  proper  thickness  has  been  obtained  and  also  to  see  that  the 
entire  section  has  a  uniform  thickness.  A  perfect  thin  section  should  be 
a  plane  parallel  plate  0.03  to  0.04  mm.  thick  and  the  surface  should  cover 
three  or  four  square  centimeters. 

When  the  section  is  finished,  it  is  carefully  cleaned  and  removed  from  its 
bed  by  heating.  A  sufficient  quantity  of  Canada  balsam  is  warmed  on  an 
object  glass  and  the  section  is  pushed  on  to  this  and  worked  around  until 
the  balsam  is  free  from  air  bubbles.  Then  a  cover-glass  about  0.10  to  0.15 
mm.  thick  is  cemented  over  it  with  Canada  balsam.  Great  care  must  be 
used  to  remove  all  the  air  bubbles  from  the  balsam  and  also  not  to  break  the 
section  in  transferring  it  or  in  pressing  the  cover-glass  on  it.  The  whole 
process  of  making  sections  requires  much  care  and  attention,  and  is  ex- 
tremely tedious,  especially  for  one  who  has  had  very  little  practice.  Even 
with  as  much  care  as  is  possible,  it  is  difficult  to  prepare  a  section  of  the 
proper  thickness  large  enough  to  be  of  much  use,  until  the  operator  has 
had  considerable  experience.  Since  this  involves  the  loss  of  a  great  amount 
of  time,  the  average  petrographer  rarely  acquires  sufficient  skill  to  make  a 
good  section. 

The  preparation  of  slides  from  schistose  or  clastic  rocks,  or  from  any  loose 
fragmental  rock  is  especially  difficult.  It  often  is  a  very  tedious  operation, 
and  the  results  may  be  ruined  by  an  intimate  mixing  of  the  abrasive  agent 
with  the  rock.  It  is  absolutely  impossible  to  clean  such  a  section.  To  be 
sure,  it  is  possible  to  fill  the  capillary  spaces,  which  exist  in  such  rocks,  by 
treating  the  fragment  from  which  the  section  is  to  be  cut,  with  a  dilute 
solution  of  mastic  or  saponlac  for  a  long  time  under  pressure.  However, 
the  organic  substance,  thus  introduced,  may  be  dissolved  out  again  by  the 
Canada  balsam  so  that  extreme  precaution  is  necessary  in  the  subsequent 
operations. 

Thin  sections  are  used  not  only  for  microscopical  investigations,  but  very 
frequently  also  chemical  tests  may  be  made  upon  them.  If  the  section  is 
covered,  a  portion  of  the  cover-glass  is  cut  with  a  diamond  glass  cutter,  which 
is  a  stylus  with  a  small  cleavage  piece  of  diamond  fastened  in  the  end,  and 
removed  by  warming  it  slightly.  The  balsam  still  adhering  to  the  uncovered 
portion  must  be  carefully  washed  away  with  benzol  or  alcohol  so  that  the 


PREPARATION  OF  MATERIAL  147 

chemical  reagents  may  have  easy  access  to  the  rock.  For  other  investiga- 
tions the  rock  section  may  be  cut  with  a  diamond  stylus  and  a  portion 
removed  from  the  object  glass  by  warming  slightly  so  that  it  is  isolated  from 
everything  else.  After  it  has  been  thoroughly  cleaned  in  benzol  or  alcohol, 
it  may  be  digested  in  acid  or  ignited  on  a  platinum  foil,  as  the  case  requires. 
Sections  of  individual  minerals  may  be  removed  from  such  a  rock  section 
and  they  can  be  investigated  micro-chemically. 


CHAPTER  VIII 
Methods  of  Separation 

All  methods  employed  to  resolve  a  rock  into  its  components  or 
to  isolate  a  single  rock  constituent  are  designated  as  methods  of 
separation.  Such  processes  are  for  various  purposes.  In  some 
cases  a  rock  constituent,  whose  accurate  determination  is  not 
possible  under  the  microscope,  is  isolated  from  the.  rock,  and  can 
then  be  investigated  further,  and  a  chemical  analysis  made.  In 
other  cases  it  is  interesting,  as  well  as  profitable,  to  determine  the 
proportional  amounts  of  the  several  constituents  of  a  rock. 
Finally,  those  minerals,  which  are  only  sporadically  present,  may 
be  concentrated  and  important  clues  to  the  genetic  relationships 
of  the  rocks  concerned  may  be  obtained  by  studying  them. 

The  methods  of  separation  generally  give  good  qualitative 
results,  but  it  often  requires  a  combination  of  several  of  these 
methods  to  isolate  a  constituent  for  quantitative  analysis,  be- 
cause the  material  must  then  possess  a  high  degree  of  purity. 

As  mentioned  above  these  methods  are  divided  into: 

1.  Chemical  methods  of  separation. 

2.  Physical  methods  of  separation. 

i.  Chemical  Methods  of  Separation 

Chemical  methods  of  separation  are  for  the  most  part  quite 
simple.  They  are  based  upon  the  different  solubilities  of  various 
minerals  in  chemical  reagents,  but  such  differences  are  not  so 
great  that  one  rock  constituent  may  be  completely  dissolved 
before  the  others  show  signs  of  solution.  These  methods,  there- 
fore, require  great  care  and  skill  in  order  to  obtain  satisfactory 
results.  Besides  these  difficulties,  a  part  of  the  rock  mass  is 
always  rendered  unavailable  for  further  investigation  and  a  large 
amount  of  by-products,  which  must  be  removed,  is  precipitated 
in  the  course  of  chemical  separations.  Such  methods  become 
so  involved  in  their  application  that  they  should  be  employed 
only  under  especially  favorable  conditions,  where  the  physical 
methods,  to  be  described  later,  are  not  effective. 

148 


METHODS  OF  SEPARATION  149 

In  some  cases  acids,  in  others  alkalies,  are  used  for  these  separa- 
tions, which  maybe  carried  out  at  ordinary  or  at  elevated  tempera- 
tures according  to  the  conditions.  High  pressure  may  be 
necessary  in  some  instances,  and  occasionally  a  molten  solvent 
may  be  found  quite  valuable.  The  most  important  reagents  are : 

1.  Hydrofluoric  Acid. — In  general,  hydrofluoric  acid  attacks 
quartz  and  silicates  high  in  silicic  acid  much  more  readily  than 
it  does  those  low  in  silicic  acid.  Among  the  more  important 
rock-forming  minerals,  anatase,  andalusite,  cyanite,  hematite, 
corundum,  magnetite,  perovskite,  rutile,  sillimanite,  spinel,  brit- 
tle micas,  staurolite,  topaz,  tourmaline,  cassiterite,  and  zircon 
are  not  attacked  by  hydrofluoric  acid  or  a  mixture  of  it  with 
hydrochloric  acid.  Minerals  of  the  garnet  group,  micas,  horn- 
blende, pyroxenes,  titanite,  etc.,  are  attacked  with  difficulty, 
while  the  feldspars  (plagioclase  more  readily  than  orthoclase), 
leucite,  quartz  and  minerals  of  the  sodalite  group  are  easily 
attacked.  Rock  glass  is  the  most  susceptible  of  all. 

Isolation  of  completely  insoluble  rock  constituents  is  best 
carried  out  in  an  evaporating  dish  of  hammered  lead  in  which 
powdered  fluorite  may  be  treated  with  sulphuric  acid.  Thin 
layers  of  the  rock  powder  to  be  treated  are  spre'ad  over  one 
another  separated  by  layers  of  fluorite  and  the  whole  mass  is 
moistened  with  dilute  sulphuric  acid  or  hydrochloric  acid.  A 
tight  cover  is  placed  on  the  dish  and  all  joints  are  sealed  with 
wax  or  some  other  cement.  The  preparation  is  left  standing 
until  the  hydrofluoric  acid  evolved  has  acted  upon  the  rock 
powder.  A  crust  of  salts  forms  very  rapidly  on  the  mineral  mass 
during  the  process  and  it  must  be  destroyed  from  time  to  time. 
The  reaction  usually  proceeds  quite  slowly,  but  it  can  be  accel- 
erated considerably  by  warming  the  lead  dish  a  little.  The 
advantage  of  this  process  is  that  large  quantities  of  material  can 
be  used,  which  is  of  special  importance  in  the  isolation  of  constit- 
uents occurring  in  very  small  quantities. 

In  other  cases  the  rock  powder  may  be  poured  slowly,  under 
constant  stirring,  into  a  concentrated  solution  of  hydrofluoric 
acid,  but  naturally  great  precaution  must  be  taken  for  protection 
from  the  vapors  evolved.  It  is  advisable  to  place  the  platinum 
dish,  in  which  such  a  process  is  carried  on,  in  a  cold  water  bath 
or  a  freezing  mixture  until  the  first  violent  reaction  has  moder- 
ated, because  the  action  of  hydrofluoric  acid  on  such  powders  fre- 
quently causes  a  decided  increase  in  temperature.  The  reaction 


150  PETROGRAPHIC  METHODS 

is  interrupted  at  the  proper  moment  by  the  addition  of  a  large 
excess  of  water.  The  residue  must  be  washed  thoroughly 
and  the  gelatinous  silica,  formed  during  the  process,  separated 
from  the  unaltered  mineral  grains  by  pressing  first  with  a  plati- 
num spatula  and  finally  with  the  fingers.  It  is  poured  off  with 

the  wash  water. 

* 
A  quantitative  separation  of  the  gelatinous  silica  cannot  be 

made  in  this  manner  and  the  residue  must  either  be  heated  to 
change  the  balance  of  the  silica  into  a  powder  that  can  be  washed 
away,  or,  after  thorough  washing,  it  may  be  treated  with  an 
alkali,  which  dissolves  the  separated  silica.  Hydroxide  of  iron 
is  generally  precipitated  in  this  process  and  must  be  extracted 
with  hydrochloric  acid.  If  a  small  quantity  of  silica  still  remains, 
the  treatment  is  repeated  until  the  material  appears  sufficiently 
pure  when  examined  under  the  microscope. 

2.  Hydrofluosilicic  Acid. — Pure  hydrofluosilicic  acid  does  not 
attack  quartz.     It  can,  therefore,  be  used  as  a  means  for  the 
quantitative    separation    of    quartz    from    a    rock.     However, 
secondary  reactions  take  place  producing  a  solvent  for  quartz 
so  that  a  considerable  portion  of  it  is  dissolved. 

3.  Hydrochloric,  Chlorous  and  Organic  Acids. — Dilute  hydro- 
chloric acid  is  used  to  isolate  silicates  from  carbonate  rocks.    How- 
ever, if  the  silicates  occurring  in  a  granular  limestone  are  easily 
attacked  by  hydrochloric  acid,  as  e.g.,  forsterite  and  gehlenite, 
strong  organic  acids  such  as  acetic  and  citric  are  recommended. 
Even  these  do  not  always  produce  favorable  results  because  they 
also  attack  silicates  considerably.    An  aqueous  solution  of  chlorous 
acid  is  the  best  reagent  to  use  in  such  cases.     It  is  best  to  prepare 
this  solution  fresh  by  treating  a  solution  of  barium  chlorate  with 
the  calculated  amount  of  dilute  sulphuric  acid,  care  being  taken 
to  avoid  an  increase  in  temperature.     After  removing  the  pre- 
cipitate of  barium  sulphate,  the  clear  filtrate  is  used.     It  attacks 
calcite  quite  energetically  and  has  no  effect  upon  most  of  the 
silicates.     Being  an  oxidizing  agent  it  may  also  be  used  for 
removing  organic  substances  from  rocks. 

Pure,  warm  hydrochloric  acid  can  be  employed  and  by  a 
skillful  modification  of  the  process,  it  is  frequently  possible  to 
remove  the  calcite  before  any  effect  on  the  silicates  is  noticeable. 
Usually,  better  results  are  obtained  than  with  organic  acids.  The 
carbonates  are  soluble  in  hydrochloric  acid  with  evolution  of  car- 
bon dioxide;  the  sulphides  with  the  development  of  hydrogen 


METHODS  OF  SEPARATION  151 

sulphide,  and  usually  with  the  separation  of  sulphur.  Leucite 
dissolves  with  the  formation  of  powdery  silica,  while  scapolite 
and  basic  plagioclase  are  more  difficultly  soluble  under  the  same 
conditions.  Minerals  of  the  sodalite  group,  nepheline,  melilite, 
olivine,  wollastonite,  and  most  of  the  zeolites  dissolve  with  the 
formation  of  gelatinous  silica,  while  serpentine  and  chlorite  are 
very  difficultly  soluble.  Apatite,  powdered  hematite  and  magne- 
tite dissolve  without  any  formation  of  by-products.  Magnetite 
dissolves  most  readily  after  addition  of  potassium  iodide. 

Concentrated  hydrochloric  acid  is  used  to  separate  various 
difficultly  soluble  silicates  from  each  other.  However,  a  perfect 
quantitative  separation  cannot  often  be  affected  by  such  means, 
because  the  mass  must  be  digested  at  higher  temperatures  for 
some  time  until  one  of  the  constituents  is  entirely  decomposed. 
During  this  process  the  other  minerals  have  either  been  partially 
dissolved  or  considerably  altered.  For  this  reason  the  earlier 
custom  of  dividing  a  rock  into  a  portion  soluble  in  hydrochloric 
acid  and  one  insoluble,  for  analytical  purposes,  has  been  entirely 
abandoned. 

4.  Sulphuric  Acid. — Sulphuric   acid  alone  is  rarely  used  to 
separate  rock  constituents.     Quartz  can  be  separated  quantita- 
tively from  the  main  mass  of  the  silicates  by  digesting  the  rock 
powder  with  dilute  sulphuric  acid  in  a  sealed  tube  at  a  tempera- 
ture of  150°  to  200°.     The  other  constituents  are  destroyed  by 
this  reagent.     Application  of  sulphuric  acid  in  other  cases  is 
scarcely  to  be  recommended. 

5.  Alum. — The  methods  for  separating  silicates  from  carbonate 
rocks,  mentioned  above,  usually  do  not  produce  accurate  results, 
which,  however,  can  be  obtained  by  the  use  of  an  alum  solution. 
This  dissolves  the  calcite  rapidly  and  leaves  most  of  the  other 
constituents,  even  dolomite  itself,  unattacked.    It  may  be  used  in 
cases  where  a  quantitative  determination  of  the  proportions  of 
calcite  and  dolomite  in  a  carbonate  rock  is  desired. 

6.  Sodium  Hydroxide. — Opal  can  be  dissolved  out  of  a  rock 
with  sodium  hydroxide.     This  process   proceeds  quite   rapidly 
even  at  ordinary  temperatures.     If  a  rock  powder  is  digested  for 
a  long  time  with  sodium  hydroxide  on  a  water  bath  a  large 
number  of  the  silicates  are  materially  altered;  for  example,  the 
feldspars  are  changed  into  compounds  easily  soluble  in  hydro- 
chloric acid.     Other  minerals,  as  pyroxene,  quartz,  etc.,  are  not 
altered. 


152  PETROGRAPHIC  METHODS 

Finally  some  methods,  that  are  used  only  in  special  cases,  should  be 
mentioned.  Thus  spinel  and  corundum  remain  unchanged  when  a  rock 
powder  is  decomposed  with  sodium  potassium  carbonate.  Carbonaceous 
substances  can  be  removed  from  a  rock  by  heating  in  an  oxidizing  flame. 
A  solution  of  cuprammonium  chloride  will  dissolve  metallic  iron.  Finally 
bituminous  substances  can  be  extracted  with  ether,  benzol,  etc. 

2.  Physical  Methods  of  Separation 

Physical  or  mechanical  methods  of  separation  depend  upon 
differences  in  physical  properties  of  the  minerals.  Specific 
gravity  is  by  far  the  most  important  of  these  so  that  the  methods 
dependent  upon  it  must  be  described  in  detail.  Methods  based 
upon  magnetic  differences  and  other  properties  of  the  minerals 
are  of  much  less  importance. 

Analyses  by  Washing. — The  methods  so  universally  employed  in  com- 
mercial processes  for  separating  substances  according  to  their  specific 
gravities  can  only  be  used  where  great  differences  in  specific  gravity  are 
involved,  as  in  the  separation  of  ores  from  rock  material.  Specific  gravity 
is  not  the  only  important  factor  in  this  process  of  separation  either- in  quiet 
or  circulating  water.  The  size  and  development  of  the  individual  mineral 
particles  are  of  special  significance  for,  on  the  one  hand,  fine  dust  will  remain 
suspended  for  a  much  longer  time  than  coarse  sand,  and  on  the  other  hand, 
thin  plates  will  settle  to  the  bottom  more  slowly  than  rounded  grains. 
Therefore,  the  washing  process  for  separating  rock  constituents  cannot  be 
used  except  where  the  minerals  have  very  different  habits,  as  mica  and 
quartz,  or  where  the  difference  in  specific  gravity  is  quite  considerable. 
Even  then  complete  separation  cannot  be  effected  by  washing  in  most  cases. 

The  apparatus  used  in  soil  analysis  for  separating  the  various  constituents 
according  to  their  abilities  to  remain  suspended  in  a  liquid  is  not  generally 
applicable  in  rock  investigation,  but  another  device  may  be  employed  for 
the  washing  tests.  It  consists  of  a  series  of  funnel-shaped  glass  vessels  one 
above  another,  each  terminating  at  top  and  bottom  in  a  glass  tube.  The 
powder  to  be  separated  is  placed  in  the  smallest  funnel  at  the  bottom. 
The  lower  tube  of  this  funnel  is  connected  with  a  water  tap.  A  hose  from 
the  upper  tube  leads  to  the  lower  tube  of  the  funnel  next  in  size  above  it. 
All  the  connections  are  made  in  this  manner  and  the  upper  glass  tube  of  the 
largest  funnel  at  the  top  of  the  series  is  made  to  discharge  into  a  sink.  A 
slow  stream  of  water  is  passed  through  the  apparatus  and  it  carries  the 
lighter  particles  along  with  it  and  deposits  them  in  the  funnel  above, 
where  the  eddies  are  weaker  on  account  of  the  larger  size  of  the  vessel.  An 
Eichstaedt  apparatus,  based  on  the  reduction  of  eddies  with  an  increase  of 
cross  section,  gives  similar  results. 

The  methods  used  in  washing  precious  stones  from  a  matrix  give  better 
results,  especially  after  some  practice.  A  shallow  porcelain  dish  may  be 
used  for  this  purpose,  or  better  still  a  flat  shield-shaped  safety  trough  in 
which  the  powder  is  spread  out  with  a  little  water.  The  trough  is  held  in 


METHODS  OF  SEPARATION  153 

one  hand  and  is  jarred  by  sharp  quick  taps  with  the  other.  The  individual 
minerals  become  separated  from  each  other  in  sharply  denned  zones.  A 
percussion  table,  such  as  is  used  in  ore  dressing,  can  also  be  constructed, 
and  operated  most  easily  by  hydraulic  pressure.  Serviceable  results  may 
be  obtained  occasionally  by  blowing  or  sucking  with  a  regulated  stream  of 
air.  Mica  and  graphite  are  but  slightly  affected  by  crushing,  while  other 
constituents  are  reduced  to  a  fine  dust  that  can  be  blown  away  by  this 
method. 

Separation  According  to  Specific  Gravity. — The  so-called 
heavy  liquids  are  used  to  isolate  the  rock  constituents  according 
to  their  specific  gravities.  This  process  is  frequently  employed, 
as  the  liquids  have  specific  gravities  higher  than  those  of  the 
individual  rock  constituents  (Table  21,  No.  11).  After  the  rock 
powder  to  be  separated  is  immersed  in  one  of  them  it  is  well 
shaken  and  allowed  to  stand  for  some  time.  The  heavier  min- 
erals sink  to  the  bottom  while  the  lighter  ones  float. 

If  the  pure  powder  of  two  different  minerals  is  well  mixed,  a 
quantitative  separation  of  each  can  be  made  by  using  a  liquid 
whose  specific  gravity  is  about  the  mean  between  those  of  the 
powders.  The  specific  gravities  of  the  two  powders  may,  how- 
ever, be  so  nearly  equal  as  to  differ  by  one  unit  in  the  second 
decimal  place. 

The  relations  are  not  so  simple  in  the  rocks.  Here  we  rarely 
have  to  deal  with  particles  that  consist  of  but  one  mineral,  as 
was  the  case  in  the  example  given,  but  the  particles  generally 
consist  of  several  minerals  intergrown  in  different  proportions. 
It  is,  therefore,  impossible  to  separate  the  individual  minerals  of 
a  rock  by  this  method.  The  separation  must  be  carried  out  in 
single  stages  so  as  to  separate  the  combinations  resulting  from 
such  intergrowth  from  the  pure  minerals.  All  this  reduces  the 
efficacy  of  the  method  so  that  great  differences  in  specific  gravity 
are  necessary  to  make  a  complete  separation.  Powders  that 
have  passed  through  a  25  to  50  mesh  sieve  are  best  for  all  these 
separations.  If  the  grains  are  much  finer,  separation  takes  place 
very  slowly  until  finally,  if  the  division  is  as  fine  as  dust,  separa- 
tion does  not  occur  at  all,  because  the  fine  particles  remain  sus- 
pended in  the  quite  immobile  liquid.  The  finest  dust  must  be 
sifted  out  or  washed  away  in  all  cases  before  the  powder  is  placed 
in  a  heavy  liquid. 

The  mobility  of  the  liquid  employed  has  a  great  deal  to  do 
with  the  rapidity  and  accuracy  of  the  separation.  Heavy 
organic  liquids,  which  may  be  used,  are  generally  quite  mobile. 


154  PETROGRAPHIC  METHODS 

On  the  other  hand,  the  heavy  solutions  of  inorganic  salts  in 
water  are  but  slightly  mobile  when  concentrated,  and  some  are 
so  viscid  that  small  particles  may  rise  or  sink  in  them  very 
slowly. 

Organic  liquids  have  an  unpleasant  odor  and  evaporate  quite 
rapidly,  but  do  not  act  upon  the  minerals  to  be  investigated. 
Inorganic  solutions  are  odorless,  attack  certain  minerals  quite 
strongly,  also  the  skin,  and  are,  for  the  most  part,  quite  poison- 
ous. Thus  each  has  its  advantages  and  disadvantages. 

To  be  of  value  for  separations  according  to  specific  gravity, 
liquids  must  be  capable  of  being  easily  evaporated  and  concen- 
trated so  that  every  gradation  of  density  can  be  readily  produced. 
In  general  the  process  is  as  follows :  grains  of  the  heavier  and  those 
of  the  lighter  rock  minerals  are  picked  out  of  the  powder.  They 
are  placed  in  the  concentrated  heavy  liquid.  The  concentration 
is  reduced,  by  dilution  with  water  in  the  case  of  inorganic  solu- 
tions and  with  ether  or  benzol  in  the  case  of  organic  liquids,  until 
some  of  the  grains  sink  while  the  others  float.  The  whole  mass 
of  rock  powder  is  now  put  into  the  liquid  thus  prepared.  Another 
method  of  obtaining  the  proper  density  of  the  heavy  liquid 
consists  in  the  use  of  a  scale  of  indicators  of  known  density. 
The  density  of  the  minerals  to  be  separated  must  also  be  known. 
Two  of  the  minerals  from  the  scale,  corresponding  in  specific 
gravity  to  the  minerals  to  be  investigated,  are  put  in  the  liquid, 
and  then  it  is  diluted  until  one  sinks  while  the  other  floats.  The 
following  list  of  test  minerals  has  been  found  to  be  quite  practical: 

Sp.  gr.  Sp.  gr. 

1.  Sulphur,  Girgenti 2 . 070     11.  Quartz,  St.    Gotthard 2 . 650 

2.  Hyalite,  Waltsch 2 . 160  12.  Labradorite,  Labrador  ....  2 . 689 

3.  Opal,  Scheiba 2.212     13.  Calcite,  Rabenstein 2.715 

4.  Natrolite,  Brevik 2.246  14.  Dolomite,  Muhrwinkel ...   2.733 

5.  Pitchstone,  Meissen 2 . 284     15.  Dolomite,  Kauris 2 . 868 

6.  Obsidian,  Lipari 2 . 362     16.  Prehnite,  Kilpatrick 2 . 916 

7.  Pearlite,  Hungary 2 . 397     17.  Aragonite,  Bilin 2 . 933 

8.  Leucite,  Vesuvius 2.465     18.  Tremolite,  Zillertal 3.020 

9.  Adularia,  St.  Gotthard...  2.570  19.  Andalusite,  Bodemais....   3.125 
10.  Nepheline,  Brevik 2 . 617  20.  Apatite,  Ehrenfriedersdorf  3 . 180 

Better  results  will  be  obtained  in  all  separations  with  heavy 
liquids  if  the  dimensions  of  the  grains  are  about  equal.  This 
method  fails  completely  with  thin,  scaly  minerals  like  mica,  for 
a  portion  of  the  mica  will  be  found  with  each  of  the  separated 
parts.  Mica  scales  can  be  removed  by  allowing  the  powder  to 


METHODS  OF  SEPARATION 


155 


glide  over  an  inclined  plane  of  rough  paper,  or  better  by  breathing 
into  quite  a  large  funnel  and  causing  the  rock  powder  to  move 
over  the  moistened  walls;  then  mica  scales  will  stick  to  the  walls 
and  can  be  removed  later  by  inverting  the  funnel  and  tapping 
it.  This  manipulation  must  be  repeated  several  times. 

Separations  with  heavy  liquids  can  be  carried  out  in  any  kind  of  a  vessel, 
but  a  beaker  is  the  best  and  simplest.  Comparatively  large  quantities  of 
the  liquid  must  be  used  and  this  is  always 
very  expensive.  It  is  also  difficult  to  re- 
move entirely  the  light  mineral  grains 
floating  on  the  surface  of  the  liquid, 
before  the  heavy  residue  is  poured  out. 

Under  certain  conditions,  especially 
when  large  quantities  of  heavy  constit- 
uents are  to  be  treated,  the  separation 
can  be  made  more  rapidly  in  a  beaker  than 
in  a  separating  funnel,  because  the  large 
mass  of  powder  sinking  to  the  bottom  of 
the  funnel  easily  clogs  up  the  delivery  tube. 
The  beaker  is  filled  over  two- thirds  full  of 
the  liquid,  whose  specific  gravity  has  been 
accurately  fixed  for  the  minerals  in  ques- 
tion, and  then  the  powder  is  poured  into 
it  under  constant  stirring.  The  powder 
separates  into  two  distinct  parts.  The 
beaker  is  then  set  in  a  large  porcelain 
evaporating  dish  and  left  to  stand  until  a 
layer  of  clear  liquid  appears  between  the 
floating  material  and  that  on  the  bottom. 
A  hollow  conical  vessel,  a,  Fig.  169,  is  set 
into  the  liquid  and  held  in  a  vertical  posi- 
tion by  a  clamp  around  the  neck  b.  A 
small  funnel,  c,  rests  in  the  upper  end  of 
the  neck.  A  large  dropping  funnel,  d,  is 
so  arranged  above  this  that  the  mercury, 
which  it  contains,  can  drop  into  the  small 
funnel.  As  the  mercury  flows  slowly  into 
the  conical  vessel,  the  latter  gradually  sinks 

until  the  beaker  overflows.  Then  the  influx  of  mercury  is  increased  so  that 
the  vessel  sinks  very  rapidly.  This  causes  the  floating  material  to  be 
washed  over  quantitatively  into  the  porcelain  dish  below. 

It  is  generally  better  to  carry  out  the  separations  in  a  separat- 
ing funnel  which  can  be  arranged  in  various  ways.  The  funnel 
sketched  in  Fig.  170  has  a  very  simple  but  practical  form.  It 
can  be  used  in  nearly  all  cases  so  that  it  will  not  be  necessary  to 
describe  more  complicated  devices  constructed  for  this  purpose. 


FIG.  169. — Apparatus  for  Separating 
Large  Amounts  of  Rock  Powder. 


156  PETROGRAPHIC  METHODS 

The  stopcock  B  is  closed  and  the  funnel  filled  up  to  E  with  the 
liquid.  Then  the  stopcock  A  is  closed  and  the  powder  poured 
into  the  upper  funnel.  After  shaking  vigorously  the  stopcock 
A  is  opened  again,  whereupon  the  heavier  constituents  collect  in 
D.  Then  A  is  closed  again  and  the  process  repeated 
until  the  separation  is  complete. 

Organic  liquids,  aqueous  solutions  of  inorganic 
salts,  and  molten  inorganic  salts  may  be  used  for 
separations  depending  upon  specific  gravity. 

Heavy  Organic  Liquids. — Many  organic  liquids  are 
characterized  by  a  high  density.  Only  those  liquids 
are  applicable  for  these  processes  that  do  not  have 
an  injurious  effect  upon  the  operator,  do  not  evap- 
orate too  easily,  and  are  quite  permanent  and  not 
too  expensive. 

Tetrabromacetylene  and  methylene  iodide  have  been 
FIG.  170.       found  to  be  extremely  useful.     When  pure,  they  are 

Separating 

Funnel.  light  yellowish,  exceptionally  mobile  liquids,  that 
can  be  diluted  easily  to  any  proportions  by  the  addi- 
tion of  ether  or  benzol.  Since  the  diluent  is  very  volatile  it  can 
be  readily  removed  from  the  liquid,  thus  concentrating  it.  Still 
there  is  always  a  considerable  loss,  because  some  of  the  heavy 
liquid  is  evaporated  along  with  the  diluent.  This  makes  such 
work  quite  expensive,  especially  when  methylene  iodide  is  used. 
The  loss  is  of  less  importance  with  tetrabromacetylene  because 
the  operator  can  prepare  it  himself. 

Tetrabromacetylene  is  an  almost  colorless  liquid  that  can  be 
very  easily  prepared  at  any  time.  Pure  acetylene  is  passed  into 
bromine  until  the  whole  solution  becomes  light  yellow.  It  is 
very  stable  and  has  no  destructive  effect  upon  the  rock  constit- 
uents. When  concentrated  its  specific  gravity  is  only  3.0,  so 
that  its  application  is  quite  limited. 

Methylene  iodide  is  distinguished  from  tetrabromacetylene 
by  a  much  higher  specific  gravity,  viz.,  3.32  at  16°  C.  It  is  con- 
siderably above  that  of  most  of  the  rock-forming  minerals.  The 
density  increases  quite  rapidly  with  a  decrease  in  temperature 
until  at  its  freezing-point,  5°  C.,  the  specific  gravity  is  3.35. 
Methylene  iodide  is  decomposed  quite  readily  even  by  the  action 
of  light,  and  this  is  a  great  disadvantage.  Iodine  separates  out 
and  colors  the  liquid  brown.  It  can  be  removed  by  shaking  the 
solution  alternately  with  potassium  hydroxide  and  water  after 


METHODS  OF  SEPARATION  157 

which  it  can  be  dried  with  calcium  chloride  and  filtered.  An- 
other method  of  purifying  it  is  to  reduce  the  temperature  a  little 
lower  than  5°  C.  and  allow  the  liquid  to  freeze.  Light  colored 
crystals  of  methylene  iodide  are  formed  and  these  are  separated 
from  the  dark  brown  liquid  by  squeezing.  Still  this  process  is 
very  expensive  because  methylene  iodide  is  so  high  priced  and 
some  is  always  lost  in  the  process.  A  powder  that  has  been 
treated  with  methylene  iodide  should  be  washed  with  ether  and 
the  washing  must  be  continued  a  long  time  before  every  trace 
of  iodine  disappears. 

The  specific  gravity  can  be  materially  increased  by  dissolving 
iodine  in  the  methylene  iodide.  It  retains  its  mobility,  but  loses 
its  transparency  by  this  process.  If  iodoform  instead  of  iodine 
is  dissolved  in  the  liquid,  a  strong  smelling,  transparent,  yellow 
liquid  is  obtained  with  a  specific  gravity  3.45.  However,  it  can 
only  be  diluted  with  pure  methylene  iodide.  Finally  iodine 
may  be  added  to  this  solution  and  a  liquid  is  obtained  which  is 
quite  mobile,  but  nontransparent  and  has  a  specific  gravity  as 
high  as  3.6. 

The  high  index  of  refraction  of  methylene  iodide  (n  =  1.75) 
makes  it  very  valuable  for  optical  determinations.  This  value 
is  increased  by  the  fact  that,  upon  addition  of  iodoform  or 
sulphur,  the  index  can  be  increased  to  1.78.  It  cannot  be  used 
with  a  Bertrand  or  an  Abbe  .total  reflectometer  because,  like  all 
liquids  containing  iodine,  it  attacks  the  strongly  refracting  glass 
used  on  these  instruments. 

Heavy  Solutions. — One  of  the  first  liquids  used  to  separate 
rock  minerals  was  a  solution  of  acid  mercuric  nitrate  in  water 
with  a  specific  gravity  of  3.3  to  3.4.  It  was  recommended  by 
Count  Schaffgotsch.  It  has  a  destructive  effect  on  a  number  of 
the  minerals  because  of  its  acid  properties  and  has  therefore  a 
limited  application.  The  loss  of  expensive  material  may  be 
reduced  to  a  minimum  by  exercising  some  care  in  working  with 
heavy  solutions.  Since  distilled  water  is  used  for  dilution  and 
washing,  little  or  no  expense  is  involved.  Work  with  heavy 
solutions  is  less  expensive  under  all  conditions  than  work  with 
organic  liquids.  On  the  other  hand,  the  greater  viscosity  of  the 
concentrated  heavy  solutions  must  be  considered.  The  solutions 
generally  used  are : 

1.  Potassium  Mercuric  Iodide,  Thoulei  Solution. — A  solution 
of  potassium  mercuric  iodide,  HgI2.2KI,  is  the  most  convenient  of 


158  PETROGRAPHIC  METHODS 

the  various  heavy  solutions  because  it  can  be  easily  prepared  and 
can  be  diluted  or  concentrated  to  any  desired  density.  It  is 
made  by  dissolving  potassium  iodide  and  red  mercuric  iodide 
in  the  ratio  of  4:5  in  water,  and  after  the  addition  of  a  little 
metallic  mercury  it  is  evaporated  on  a  water  bath  (not  over  a 
direct  flame!)  until  a  fragment  of  fluorite  (sp.  gr.  =3.15)  floats  on 
it.  Several  needle-like  crystals  form  upon  cooling  and  the  clear 
yellow  transparent  liquid  has  a  specific  gravity  of  nearly  3.2 — in 
summer  only  3.17.  It  is  not  very  mobile  when  concentrated, 
but  it  can  be  diluted  with  water  to  any  extent  and  concentrated 
again  on  a  water  bath.  A  small  excess  of  potassium  iodide 
does  no  harm  because,  upon  evaporation,  it  separates  out  as  a 
crystalline  crust  over  the  liquid,  but  an  excess  of  mercuric 
iodide  is  injurious  because  it  is  precipitated  upon  the  addition 
of  water. 

If  a  solution  of  a  definite  specific  gravity  is  to  be  prepared  by 
dilution,  it  cannot  be  done  accurately  by  adding  a  measured 
amount  of  water  to  the  concentrated  solution,  because  there  is  a 
strong  contraction  of  the  solution  when  it  is  diluted.  The  density 
of  the  prepared  liquid  must  be  checked  by  a  Westphal  balance 
or  by  an  indicator.  The  final  steps  in  fixing  the  specific  gravity 
of  a  solution  should  not  be  attempted  by  adding  water,  but  by 
adding  a  less  concentrated  solution  of  the  same  kind. 

Thoulet  solution  has  some  disadvantages  among  which  are  its 
poisonous  character  and  its  cauterizing  effect  on  the  skin.  It 
attacks  metals  vigorously  with  the  separation  of  mercury.  It  is 
also  very  difficult  to  remove  from  the  mineral  powder.  Some- 
times this  can  only  be  accomplished  by  repeated  digestion  with 
potassium  iodide  solution  and  water.  Attention  was  called  to 
its  high  index  of  refraction  in  Part  I,  page  37. 

2.  A  solution  of  cadmium  borotungstate,  Klein's  solution, 
9W03.B203.2(CdOH)  +16H2O  is  much  more  difficult  to  prepare 
and  is  usually  not  made  by  the  operator  himself.  When  a  dilute 
solution  of  it  is  evaporated  on  a  water  bath  until  a  film  of  crystals 
forms  over  it,  a  yellow  liquid  with  a  specific  gravity  of  3.36  is 
obtained  upon  cooling.  It  is  quite  viscid,  but  it  can  be  mixed 
with  water  in  any  proportions  without  decomposition,  and  its 
mobility  is  increased  very  rapidly  upon  dilution.  The  liquid  is 
harmless,  but  is  decomposed  by  carbonates  as  well  as  metals,  and 
these  must  be  removed  before  using  it.  The  crystals  of  the 
tungstate,  which  form  upon  concentration,  can  be  purified  by 


METHODS  OF  SEPARATION  159 

draining  off  the  mother  liquor.  They  melt  at  75°  C.  to  a  liquid 
whose  specific  gravity  is  3. 6,  but  it  is  so  viscid  that  it  cannot  be 
used  to  separate  fine  powders. 

3.  Barium  mercuric  iodide,  Rohrbach  solution,  is  quite  mobile 
with  a  specific  gravity  of  3.59.  It  decomposes  very  easily  so 
that  even  its  preparation  (100  parts  BaI2  + 130  parts  HgI2)  must 
be  carried  out  with  great  care.  The  concentrated  solution  can- 
not be  diluted  with  water.  A  less  concentrated  solution  must 
be  added  very  slowly  by  pouring  it  over  the  concentrated  solu- 
tion in  a  thin  layer  and  allowing  the  two  to  diffuse.  The  diffi- 
culties encountered  in  diluting  this  solution  are  so  great  that  its 
application  is  limited  and  of  late  it  is  rarely  employed. 

Heavy  Molten  Liquids. — It  is  necessary  to  work  at  high  temperatures  with 
molten  inorganic  salts.  Cadmium  borotungstate  has  already  been  referred 
to.  Other  salts  recommended  for  this  purpose  are: 

Lead  chloride,  sp.  gr.  =  5.0,  and  zinc  chloride,  sp.  gr.  =  2.4.  When  melted 
they  can  be  mixed  with  each  other  in  all  proportions.  The  melting-point 
is  about  400°  C.  The  operations  may  be  carried  on  best  in  a  small  test-tube 
on  a  sand  bath,  and  the  powder  to  be  separated  should  be  poured  into  the 
melted  mass  in  small  portions.  The  liquid  is  very  viscid  and  splutters  a 
great  deal  so  that  it  is  quite  dangerous  to  use  it.  When  the  separation  is 
completed,  the  mass  is  cooled  and  the  cake,  thus  forced,  is  cut  through  the 
middle  horizontally.  The  mineral  particles  are  freed  from  the  mass  and 
purified  by  digestion  with  water  to  which  a  little  nitric  acid  has  been  added. 
Mercurous  nitrate  melts  at  70°  C.  to  a  very  mobile  liquid  with  a  specific 
gravity  of  4.3.  It  can  be  liquefied  very  easily  on  a  water  bath  and  diluted 
in  any  proportions  with  warm  water.  It  possesses,  however,  the  very 
disagreeable  property  of  decomposing  with  a  rapid  separation  of  a  basic  salt 
which  has  a  high  melting-point.  For  this  reason  the  molten  salt  often  begins 
to  solidify  before  the  separation  of  the  rock  powder  is  complete.  If  the  melt 
is  diluted  with  water  it  is  somewhat  less  decomposable.  Molten  silver 
nitrate  is  also  very  useful.  By  heating  to  200°  C.  a  mobile  liquid  is  obtained 
with  a  specific  gravity  of  4. 1 .  This  can  be  diluted  in  all  proportions  with  potas- 
sium nitrate  and  a  rock  powder  can  be  completely  separated  in  this  liquid. 
However,  the  high  temperature  at  which  one  must  work  makes  the  use  of 
this  salt  quite  unpleasant.  Thallium  silver  nitrate  is  prepared  by  dissolving 
equivalent  amounts  of  thallium  and  silver  in  nitric  acid.  At  75°  C.  the 
double  salt  forms  a  liquid,  as  mobile  as  water,  and  has  a  specific  gravity  of 
nearly  5.0.  It  can  be  diluted  in  all  proportions  with  water  and  concentrated 
again  by  evaporation  on  a  water  bath.  It  is  necessary  to  work  very  care- 
fully with  this  melt  because  the  specific  gravity  increases  upon  cooling  as 
well  as  upon  the  evaporation  of  the  water.  The  separation  should  be 
carried  out  as  rapidly  as  possible  on  a  water  bath  at  constant  temperature. 
Then  the  mass  should  be  solidified  rapidly  by  directing  a  stream  of  cold 
water  upon  the  outside  of  the  containing  vessel.  Its  properties  are  very 
similar  to  those  of  thallium  mercurous  nitrate  with  a  specific  gravity  of  5.3, 


160 


PETROGRAPHIC  METHODS 


except  that  it  is  decomposed  by  sulphides.     Both  of  these  salts  are,  however, 
quite  expensive  on  account  of  the  high  price  of  thallium. 

Magnetic  Separation. — The  methods  of  magnetic  separation  give  good 
results  in  many  cases.  Sometimes  a  simple  magnet  and,  sometimes,  an 
electro-magnet  is  used  for  this  purpose.  A  simple  magnet  is  very  useful  to 
draw  out  steel  fragments  which  have  been  introduced  into  the  powder  dur- 
ing pulverization  and  would  decompose  Thoulet's  solution.  Besides  metal- 
lic iron,  only  magnetite  and  pyrrhotite  are  attracted  by  a  simple  magnet. 
They  are  best  removed  by  spreading  the  dry  rock  powder  out  on  a  paper 
stretched  on  a  frame  supported  by  four  legs.  Then  a  magnetized  steel  comb 
is  drawn  along  underneath  the  paper.  The  magnetic  particles  are  thus 
drawn  to  the  edge  of  the  paper. 


FIG.  171  a. — Electro-magnet. 

An  electro-magnet  has  a  much  more  extensive  application.  The  best 
form  of  this  is  a  horseshoe.  A  rectangular  piece  of  soft  iron  is  screwed  on 
each  pole  and  so  arranged  that  by  turning  the  pieces  the  distance  between 
the  poles  can  be  regulated,  Fig.  17 la.  Thus,  the  strength  of  the  electro- 
magnet can  be  controlled.  Most  of  the  minerals  containing  iron  are 
attracted  by  it,  while  those  free  from  iron — quartz,  feldspar,  etc. — remain 
behind  unless  they  contain  magnetic  inclusions.  The  degree  of  magnetic 
attraction  is  independent  of  the  amount  of  iron  present.  As  yet,  minerals 
have  not  been  systematically  arranged  according  to  the  degree  of  attrac- 
tion, but  a  short  classification  of  the  most  important  minerals  can  be  found 
in  Table  19,  No.  VI. 

To  separate  a  rock  powder  electro-magnetically,  it  is  spread  out  on  a  stiff 
piece  of  paper  and  passed  back  and  forth  under  the  poles  of  an  electro-mag- 


METHODS  OF  SEPARATION 


161 


net,  the  strength  of  the  magnet  being  regulated  as  described  above.  This 
paper  is  removed  from  time  to  time  and  a  clean  sheet  is  placed  under  the 
magnet.  Then  by  breaking  the  circuit  the  attracted  particles  fall  off. 

The  separation  can  be  made  in  water  or  alcohol  when  very  fine  powder  is  to 
be  treated.  The  rock  powder  is  washed  in  the  liquid  and  then  the  latter  is 
caused  to  pass  over  the  magnet.  The  process  can  be  accomplished  in  much 
the  same  manner  as  it  is  on  a  large  scale  in  the  commercial  separation  of 
ores.  Numerous  minerals  that  are  only  slightly  magnetic  become  strongly 
magnetic  when  heated  to  redness  in 
an  oxidizing  flame.  This  phenom- 
enon finds  extensive  application  in 
science  as  well  as  in  commercial 
processes. 

Other  Methods  of  Separation. — 
Difference  in  fusibility  has  been 
suggested  as  a  means  of  separating 
minerals,  but  this  method  will  not 
give  good  results  except  when  the 
difference  is  very  pronounced  and 
the  mineral  particles  are  of  even 
size.  The  powder  is  spread  out  oh 
a  platinum  foil  and  heated  to 
glowing  in  a  blast  flame  until  the 
more  easily  fusible  part  is  melted 
together,  then  the  unmelted  portion 
is  simply  shaken  off. 

Finally,  there  are  cases  in  which  certain  rock  constituents  must  be  sorted 
by  hand.  A  lens  is  used  in  this  work,  but  it  must  not  have  too  strong  a 
magnification  because,  on  the  one  hand,  the  short  focal  length  of  a  high 
power  objective  lowers  it  so  as  to  be  in  the  way  of  the  operator  and,  on  the 
other  hand,  the  unsteadiness  of  the  hand  is  more  evident.  The  instrument 
shown  in  Fig.  171  is  quite  useful  for  such  operations.  A  strip  of  glass  whose 
width  equals  the  diameter  of  the  lens,  can  be  pushed  back  and  forth  under 
the  lens.  The  powder  is  spread  out  evenly  on  this  glass  by  means  of  a 
distributer  and  is  observed  through  the  lens.  The  individual  grains  are 
then  removed  with  a  pair  of  small  forceps  or  a  sharp  moistened  splinter  of 
soft  wood.  The  powder  can  also  be  immersed  in  a  liquid  on  the  glass  strip 
to  make  the  grains  more  transparent.  It  is  often  advantageous  to  use  a 
pointed  glass  tube  attached  to  a  suction  pump  to  remove  the  desired  grains. 
These  are  drawn  through  the  tube  over  into  a  glass  flask  arranged  for  this 
purpose.  With  a  little  practice  very  fine  rock  particles  can  be  sorted  out 
with  this  apparatus.  It  can  also  be  used  to  examine  the  particles  in  polar- 
ized light  by  placing  a  glass  plate  at  the  proper  angle  below  the  stage  and  a 
nicol  prism  above  the  lens.  In  this  way  the  apparatus  has  an  extensive 
applicability.  A  binocular  microscope,  Part  I,  page  41,  also  gives  good 
results  in  such  cases. 


FIG.  171.— Lens  Stand.     (W.  and  H.  Seibert.) 


11 


CHAPTER  IX 
Methods  of  Investigation 

Investigation  with  a  polarizing  microscope  is  the  most  impor- 
tant and  most  frequently  used  of  all  the  methods  of  petrograph- 
ical  research.  Next  in  importance  to  the  microscope  are  the 
quantitative  and  qualitative  chemical  analyses  which  enable 
one  to  make  as  complete  a  determination  of  a  rock  as  does  an 
optical  examination  of  a  thin  section.  A  determination  of  the 
specific  gravity  and  other  physical  properties  is  also  made  to 
complete  the  information  concerning  a  mineral. 

The  methods  of  investigation,  like  those  of  separation,  fall 
naturally  into  two  groups.  These  are  the  chemical  and  physical 
methods.  The  treatment  here  includes  only  certain  subordinate 
sections  of  each  because  the  general  chemical  methods  are  not 
within  the  province  of  this  book,  while  the  optical  methods  have 
been  thoroughly  discussed  in  Part  I. 

Under  the  heading  of  chemical  methods,  only  such  chemical 
reactions  are  discussed  as  are  adapted  to  a  rapid  recognition  of 
minerals  in  the  simplest  way  possible,  especially  with  very  small 
amounts  of  the  material.  Special  reactions,  which  are  very 
characteristic  for  certain  minerals,  will  also  be  given.  The 
physical  part  includes  all  physical  methods  except  the  optical 
investigations. 

i.  Chemical  Methods  of  Investigation 
(a)  General  Reactions 

The  methods  of  micro-chemistry  may  be  mentioned  first,  as  of 
importance  in  the  general  reactions  of  a  number  of  the  elements. 
These  can  only  be  applied  successfully  when  there  is  nothing  to 
interfere  with  them.  In  many  cases  the  tests  are  so  simple  and 
easy  that  good  results  may  be  obtained  with  them,  but  in  just  as 
many  other  cases  they  are  very  complicated  and  the  obtaining  of 
unquestionable  results  is  not  dependent  upon  a  large  amount  of 
practice  alone  but  also  upon  numerous  other  contingencies,  the 

162 


METHODS  OF  INVESTIGATION 


163 


efficaciousness  of  which  cannot  be  positively  determined  in  every 
case.  Only  such  of  the  numerous  micro-chemical  methods  will 
be  discussed  as  are  comparatively  simple  and  give  definite 
results. 

Reactions  with  Fluosilicic  Acid. — The  reactions  with  fluosilicic  acid,  which 
is  replaced  by  hydrofluoric  acid  in  the  investigation  of  silicates,  are  especially 
important  for  petrographical  studies  because  the  fluosilicates  formed  are 
very  soluble  and  crystallize  easily.  They  do  not  possess  the  importance 
generally  ascribed  to  them  because,  in  addition  to  the  characteristic  salt  of 
any  base,  frequently  double  salts  also  separate  out,  and  the  composition  of 
these  as  well  as  the  conditions  under  which  they  form  have  not  as  yet  been 
thoroughly  studied.  They  do  not  seem  to  give  good  results  in  the  investi- 
gation of  minerals  like  the  feldspars  containing  aluminium  and  alkalies 


FIG.  172. — Sodium  Fluosilicate. 


FIG.  173. — Calcium  Fluosilicate. 


because  a  double  salt  is  always  formed  in  varying  amounts  and  its  exact 
composition  is  unknown.  These  investigations  are  carried  out  on  an  object 
glass  covered  with  a  uniform  thin  layer  of  Canada  balsam  free  from  bubbles 
and  not  completely  hardened.  It  can  be  prepared  by  placing  some  balsam 
on  a  glass  and  warming  it  slightly.  A  celluloid  plate  can  also  be  used,  but 
celluloid  always  shows  some  double  refraction  phenomena,  and  this  inter- 
feres with  the  optical  investigation.  A  drop  of  pure  fluosilicic  acid  (with 
silicates  pure  hydrofluoric  acid  suffices)  is  placed  on  the  mineral  splinter 
and  the  solution  is  left  in  a  closed  box  of  dry  wood  to  evaporate  completely. 
The  crystals  are  formed  first  toward  the  end  of  the  process.  Every  trace  of 
acid  must  be  positively  absent  before  investigating  under  a  microscope,  or 
there  is  danger  of  etching  the  objective.  The  fluosilicates  of  sodium, 
potassium,  and  calcium  have  higher  indices  of  refraction  than  Canada ' 
balsam,  while  the  indices  of  magnesium,  iron,  and  manganese  fluosilicates 
are  lower. 

Sodium fluo silicate,  hexagonal;  low  double  refraction,  negative;  mostly  in 
short,  thick  prisms  and  twins  as  shown  in  Fig.  172,  or  in  long  needle-like  or 
pyramidal  crystals  rarely  tabular,  parallel  to  the  base.  Potassium  fluosilicate, 


164  PETROGRAPHIC  METHODS 

cubes  and  octahedrons,  usually  quite  large ;  often  distorted,  tabular  octahed- 
rons, quite  similar  to  the  plates  of  sodium  fluosilicate  mentioned  above.  Lith- 
ium fluosilicate,  not  very  characteristic ;  acicular  and  monoclinic,  apparently 
hexagonal.  Magnesium  fluosilicate,  as  well  as  the  similar  compounds  of  iron, 
manganese,  nickel  and  cobalt :  rhombohedral,  colorless  or  very  light  colored ; 
short  to  long  prismatic,  often  with  many  faces;  high  double  refraction, 
positive;  characterized  by  brilliant  interference  colors.  Calcium  fluosili- 
cate, short,  monoclinic  prisms  with  inclined  terminations,  Fig.  173,  low 
double  refraction,  negative;  6  =  0,  c  :  B  =  40° /.  Irregular  growths  with 
rounded  outline  are  generally  observed;  crystals,  rare.  The  crystals  of 
strontium  and  barium  fluo silicates  are  quite  similar  to  these.  The  latter  are 
usually  very  small,  since  they  are  quite  insoluble.  Aluminium  fluosilicate 
is  a  gelatinous  substance.  It  forms  rhombohedral  double  salts  with  the 
alkalies  and  these  salts  have  strong  double  refraction. 

Reactions  with  Certain  Elements. — For  these  reactions  the 
minerals  to  be  tested  are  dissolved  in  sulphuric  or  hydrochloric 
acid,  or  they  can  be  fused  in  a  platinum  crucible  with  sodium 
potassium  carbonate  and  a  small  amount  of  saltpeter  or  potas- 
sium acid  sulphate.  Sometimes  potassium  fluoride  or  fluorite 

is  added.  The  salts  obtained 
from  these  operations  may 
then  be  further  studied. 
Flame  or  blowpipe  reactions 
give  more  definite  results  in 
many  cases  than  micro-chem- 
ical tests,  the  latter  being 
used  in  special  cases.  The 
most  important  and  reliable 
reactions  that  will  be  consid- 
ered are  the  following: 

Sodium. — The  yellow  color 
of  the  sodium  flame  serves  as 
FIG.  174.— Sodium  Uranyi  Acetate.  an  indication  of  this  element. 

The  slightest  trace  of  sodium 

impurities  will  produce  the  yellow  flame,  hence  it  is  better  to  make 
a  micro-chemical  test  for  the  sake  of  accuracy.  A  drop  of  an 
acetic  acid  solution  (N.  B.  free  from  sodium!)  of  uranyl  acetate 
is  added  to  the  solution  to  be  investigated.  Sharply  defined, 
light  yellow  tetrahedrons  of  sodium  uranyl  acetate  are  formed 
with  an  index  of  refraction  less  than  that  of  Canada  balsam. 
Fig.  174  shows  these  crystals  under  the  microscope  with  the 
tube  slightly  raised.  If  magnesium,  or  a  similar  element,  is 
present  in  the  solution,  a  triple  acetate  containing  all  these  ele- 


METHODS  OF  INVESTIGATION  165 

merits  is  formed.  It  is  easily  obtained  in  large  rhombohedral 
crystals  of  very  light  yellow  color,  low  index  of  refraction,  and 
very  high  negative  double  refraction.  It  generally  forms  com- 
binations similar  to  a  cubic  icositetrahedron.  If  there  is  but  a 
small  amount  of  sodium  present,  some  magnesium  salt  can  be 
added,  if  not  already  present,  and  a  very  sensitive  reaction  for 
sodium  can  be  obtained  with  the  triple  acetate,  since  it  contains 
only  1.5  per  cent,  sodium. 

Potassium. — The  violet  colored  flame  of  potassium  is  obscured 
by  sodium  if  it  is  present  at  the  same  time.  The  potassium  color 
can  be  recognized  when  the  colored  flame  is  observed  through  a 
blue  cobalt  glass,  but  numerous  potassium  silicates  do  not  give 
the  characteristic  coloration.  These  must  be  heated  with 
gypsum,  free  from  alkali,  or  with  a  mixture  of  gypsum  and  fluor- 
ite.  Potassium  is  best  precipitated  as  potassium  platinochloride 
which  begins  to  crystallize  only  after  a  long  time  has  elapsed. 
Crystallization  can  be  accelerated  by  adding  a  drop  of  alcohol. 
Sharp,  yellow  octahedrons  and  combinations  of  it  with  the  cube 
are  formed,  which  have  a  very  high  index  of  refraction.  Am- 
monium, rubidium  and  caesium  give  rise  to  the  same  type  of 
crystals. 

Lithium. — The  carmine  red  colored  flame  of  lithium  is  obscured  by  sodium. 
Nevertheless  a  very  small  quantity  of  lithium  can  be  recognized  in  the 
presence  of  a  predominating  amount  of  sodium  in  an  ordinary  flame.  The 
powdered  mineral  is  stirred  by  a  heated  platinum  wire,  which  with  the 
adhering  particles  is  dipped  into  tallow.  The  lithium  color  appears  when 
this  preparation  is  burned  in  an  ordinary  candle  flame.  The  determination 
by  means  of  a  spectroscope  is  absolutely  positive,  while  micro-chemical  re- 
actions, e.g.,  precipitation  as  a  carbonate,  are  less  reliable.  If  a  little  potas- 
sium carbonate  is  added  and  the  solution  evaporated,  lithium  carbonate 
crystallizes  in  slender  needles  having  a  strong  double  refraction,  parallel 
extinction,  and  a  negative  character  of  the  principal  zone.  These  needles 
almost  invariably  unite  to  form  sheaf-like  or  plume-shaped  aggregates,  and 
are  characterized  by  a  lower  index  of  refraction  than  Canada  balsam. 

Calcium.— The  yellowish-red  flame  of  calcium  salts  is  not 
characteristic  enough.  A  micro-chemical  test  for  calcium  is 
better.  A  drop  of  very  dilute  sulphuric  acid  is  added  to  the 
solution  of  a  calcium  salt  and  upon  evaporation  slender  needles 
of  gypsum  crystallize  out,  which  tend  to  unite  into  star-  or 
sea  urchin-shaped  aggregates,  especially  in  acid  solutions.  When 
the  calcium  content  is  small,  it  is  better  to  evaporate  the  sul- 
phuric acid  off  of  the  object  glass  and  to  take  up  the  residue  with 


166  PETROGRAPHIC  METHODS 

a  drop  of  water.  Then,  the  individuals  appear  as  needles  with 
oblique  terminations  making  an  angle  of  65  1/2°,  Fig.  175.  They 
unite  very  readily  to  form  the  characteristic  swallow-tail  twins 
with  a  reentrant  angle  of  131°.  See  the  description  of  the  mineral 
gypsum  on  a  subsequent  page  for  its  optical  properties.  Calcium 
oxalate  forms  small  crystals  very  difficult  of  determination. 

Strontium  and  Barium. — The  purplish-red  flame  of  strontium  and  the 
yellowish-green  of  barium  are  not  very  reliable  tests  without  the  aid  of  a 
spectroscope,  but  good  tests  may  be  obtained  micro-chemically.  Some 
potassium  oxalate  is  added  to  a  neutral  solution  of  the  salts,  which  is  left 

to  stand  for  some  time  on  an  object 
glass,  covered  so  as  to  retard  evapora- 
tion. Barium  oxalate  crystallizes  out 
in  fan-shaped  crystal  skeletons  and 
needles  with  oblique  end  faces,  115°. 
They  have  an  extinction  c  :  fi  =24°. 
The  lowest  index  of  refraction  exceeds 
that  of  Canada  balsam  only  slightly. 
The  double  refraction  is  very  high. 
Strontium  oxalate  usually  shows  two 
types  of  crystallization,  but  four- 
pointed  stars  are  especially  charac- 
teristic. When  they  lie  horizontally, 
they  do  not  affect  parallel  polarized 
light  and  in  convergent  light  they 
FIG.  175. — Gypsum.  give  an  expanded  cross  that  shows 

positive  double  refraction.     It  is  quite 

significant  that  when  they  are  embedded  in  liquid  Canada  balsam  they 
disappear  entirely  and  cannot  be  found  again  in  polarized  light.  The  other 
type  of  crystallization  shows  small  rectangles  with  horn-like  projections 
on  the  corners.  The  crystals  have  strong  double  refraction  and  an 
index  of  refraction  somewhat  higher  than  the  other  type  of  crystals. 
Barium  oxalate  crystallizes  from  hot  solutions  in  elongated  six-sided, 
tabular  crystals  that  can  be  recognized  as  penetration  twins  of  two  or 
more  individuals.  They  are  perhaps  triclinic  with  cross  sections  like 
crystals  of  stilbite.  The  double  refraction  is  rather  low.  Strontium 
salts,  under  these  conditions,  give  small  lancet-shaped  bars  with  a  positive 
principal  zone,  parallel  extinction,  and  medium  double  refraction.  The 
index  of  refraction  is  less  than  that  of  Canada  balsam.  They  form  star- 
shaped  twins  at  an  angle  of  about  90°  and  give  no  interference  figure  in 
convergent  light.  These  tests,  as  well  as  numerous  other  micro-chemical 
tests,  are  of  little  value  to  prove  the  presence  of  these  two  alkali  earths. 

Beryllium. — There  is  no  practical  method  to  prove  absolutely  the  presence 
of  beryllium  in  small  amounts. 

Magnesium. — The  micro-chemical  test  for  magnesium  is  car- 
ried out  after  the  iron  and  manganese  salts  have  been  precipi- 


METHODS  OF  INVESTIGATION 


167 


tated  by  a  slight  excess  of  ammonia  in  the  presence  of  sal  ammo- 
niac. The  solution  is  greatly  diluted  and  sodium  phosphate 
added.  It  is  then  allowed  to  evaporate,  and  as  the  solution  is 
concentrated,  hemimorphic  orthorhombic  crystals  of  magnesium 
ammonium  phosphate,  Fig.  176,  form.  They  are  much  more 
perfect  than  when  they  are  precipitated  rapidly  by  an  excess  of 
ammonia.  They  have  a  rather  low  double  refraction,  and  lower 
indices  of  refraction  than  Canada  balsam.  The  positive  bisectrix 
has  a  somewhat  oblique  posi- 
tion in  the  trapezoidal  crys- 
tals. The  axial  angle  is  very 
small  and  the  optic  plane  coin- 
cides in  direction  with  the 
parallel  edges  of  the  crystals. 

Iron. — The  formation  of 
Prussian  blue  upon  the  addi- 
tion of  potassium  ferrocya- 
nide  to  an  oxidized  solution 
of  iron  salts  is  a  far  better 
test  for  the  presence  of  iron 
than  any  micro-chemical  reac- 
tion. Even  if  there  is  an  ex- 
ceptionally small  content  of 

iron,  the  solution  is  colored  a  distinct  green.  If  there  are  but 
traces  of  iron  in  the  solution,  the  best  test  is  to  note  the  red  color 
upon  the  addition  of  potassium  sulphocyanide. 

Nickel  and  Cobalt. — These  two  elements  are  rare  constituents  of  rock- 
forming  minerals.  Small  amounts  of  nickel  can  be  detected  by  adding 
yellow  ammonium  sulphide  to  an  ammoniacal  solution  of  nickel.  The 
liquid  becomes  brownish  if  only  a  trace  is  present.  Still  smaller  traces  can 
be  detected  by  the  rose-red  to  brown  color  resulting  upon  the  addition  of 
sodium  trithiocarbonate  to  a  nickel  solution.  Sodium  trithiocarbonate  is 
prepared  by  shaking  a  sodium  sulphide  solution  with  carbon  disulphide. 
Cobalt  must  be  removed  as  potassium  cobalt  nitrate  before  this  test  can  be 
carried  out,  because  cobalt  blackens  the  solution.  The  best  test  for  nickel 
is  the  following:  a  strong  ammoniacal  solution  is  either  violently  shaken  in 
air  or  is  boiled  with  powdered  a-dimethyl  glyoxime  when  a  scarlet  precipi- 
tate is  formed. 

The  simplest  test  for  cobalt  is  the  blue  bead  of  microcosmic  salt.  If 
potassium  nitrite  is  added  to  a  dilute  ammoniacal  solution  of  cobalt,  which 
is  then  acidified  with  acetic  acid,  small  cubes  of  deep  yellow  potassium 
cobalt  nitrite  are  precipitated.  They  often  appear  entirely  opaque  under 
the  microscope.  The  following  reaction  is  more  sensitive:  potassium 


FIG.  176. — Magnesium  Ammonium 
Phosphate. 


168  PETROGRAPHIC  METHODS 

sulphocyanide  is  added  to  a  solution  containing  cobalt  and  ethyl  alcohol 
carefully  poured  over  it.  The  alcohol  assumes  an  intense  blue  color. 

Manganese. — The  smallest  traces  of  manganese  can  be  detected  by  fusing 
the  mineral  with  sodium  carbonate  and  saltpeter  on  a  platinum  foil.  If 
there  is  considerable  manganese,  the  melted  mass  appears  green,  but  if 
the  content  of  manganese  is  quite  small  and  silica  is  present,  it  is  sky 
blue.  This  color  may  be  clearly  observed  with  extremely  small  traces  of 
manganese. 

Chromium. — The  presence  of  chromium  can  be  proved  by  fusion  in  a 
bead  of  microcosmic  salt  in  an  oxidizing  flame.  The  bead  is  red  when  hot, 
and  emerald  green  when  cold.  The  mineral  may  also  be  fused  with  sodium 
carbonate  and  saltpeter  on  a  platinum  foil  and  the  melt  dissolved  in  hot 
water.  When  the  solution  is  acidified  with  nitric  acid  and  a  drop  of  silver 
nitrate  added,  it  becomes  brilliant  red  and  deep  red  rhombohedrons  of  silver 
chromate,  with  an  angle  of  about  70°  and  a  high  index  of  refraction,  are 
formed.  With  only  traces  of  chromium  the  solution  has  a  fluorescent  yel- 
low color  like  that  characteristic  of  a  dilute  solution  of  eosine. 

Tungsten. — Minerals  containing  small  traces  of  tungsten  form  a  blue  glass 
when  fused  with  phosphorous  pentoxide  in  a  platinum  crucible. 

Molybdenum. — A  solution  of  1  gr.  phenyl  hydrazine  in  70  gr.  of  50  per 
cent,  acetic  acid  gives  a  good  reaction  for  molybdenum.  The  mineral  is 
dissolved  and  digested  two  minutes  in  the  reagent,  and  if  molybdenum  is 
present,  the  solution  becomes  red  and  can  be  decolorized  again  by  shaking 
with  chloroform. 

Cerium,  Lanthanum,  Didymimium,  Yttrium,  Erbium,  Thorium,  etc. — 
No  reactions  are  known  for  positively  detecting  the  so-called  rare  earths 
in  the  small  quantities  in  which  they  occur  in  rocks.  The  micro-chemical 
methods  that  might  be  used  do  not  give  rise  to  well  developed  crystals  and, 
furthermore,  the  difference  between  the  various  earths  has  not  been  thor- 
oughly studied.  Spectrum  analysis  appears  to  be  the  only  method  that 
affords  any  degree  of  certainty  in  the  determination  of  the  rare  earths. 
Even  by  this  method,  only  the  recognition  of  neodymium  and  praseodymium 
is  positively  certain.  The  mineral  grain  to  be  studied  is  focused  under  the 
microscope  with  such  a  magnification  that  the  grain  nearly  covers  the  whole 
field  of  vision.  Then  a  good  spectroscope  is  placed  over  the  ocular  when 
the  presence  of  neodymium  will  be  noticed  by  a  broad  absorption  band  in 
the  yellow  and  a  somewhat  narrower  one  in  the  green.  If  praseodymium  is 
present,  the  strongest  absorption  occurs  between  blue  and  violet  and  a 
weaker  one  in  the  blue.  This  test  is  used  on  rock-forming  minerals  only 
when  monazite  is  a  constituent  of  the  rock.  Monazite  generally  contains 
both  elements. 

Aluminium. — A  fragment  of  a  mineral  is  moistened  with  cobalt 
nitrate  solution  and  heated  to  redness  in  a  blowpipe  flame  in 
order  to  test  for  aluminium.  It  becomes  blue,  if  aluminium  is 
present,  but  this  color  is  easily  obscured  by  oxide  of  iron.  The 
presence  of  aluminium  can  be  proved  micro-chemically  by  adding 
slightly  diluted  sulphuric  acid  and  a  small  grain  of  some  caesium 


METHODS  OF  INVESTIGATION  169 

salt  to  a  solution  of  the  mineral,  when  well  developed  octahedrons 
of  caesium  alum  will  be  formed.  These  are  characterized  by  an 
especially  high  index  of  refraction.  If  dendrites  form  instead  of 
the  crystals  they  can  be  recrystallized  in  water  and  octahedrons 
obtained.  Ferric  salts  give  the  same  reaction,  but  the  two  can  be 
differentiated  by  treatment  with  ammonium  sulphide  vapor.  If 
larger  amounts  of  ferric  oxide  are  present,  the  reaction  is  indis- 
tinct. Iron  must  be  reduced  first  with  sulphurous  acid  before 
the  test  for  aluminium  is  made. 

Boron. — The  green  flame  is  a  sufficient  test  for  the  presence  of  boric  acid 
in  minerals.  It  is  obtained  when  the  powdered  mineral  is  mixed  with 
fluorite  and  potassium  bisulphate  in  a  platinum  crucible  and  placed  in  a 
flame — Turner's  test.  The  reaction  proceeds  better  if  the  mineral  is  decom- 
posed by  fusion  and  the  melt  dissolved  in  hydrochloric  acid.  Methyl 
alcohol  is  added  to  this  solution  and  when  ignited  it  burns  with  a  green 
flame.  For  a  micro-chemical  test,  the  mineral,  for  example  tourmaline,  is 
fused  in  a  platinum  crucible  with  sodium  carbonate,  if  it  is  too  difficultly 
soluble  in  hydrofluoric  and  sulphuric  acids.  The  fused  mass  is  then  dis- 
solved in  concentrated  sulphuric  and  hydrofluoric  acids.  A  platinum  cover 
with  a  drop  of  water  adhering  to  the  under  side  of  it  is  placed  on  the  crucible 
and  the  latter  is  warmed  until  the  acid  begins  to  vaporize.  The  drop  of 
water  takes  up  the  silicon  tetrafluoride  as  well  as  the  boron  fluoride  which 
develops.  This  solution  is  placed  on  an  object  glass  covered  with  Canada 
balsam  and  potassium  chloride  is  added.  Upon  evaporation  rhomboidal 
or  elongated  six-sided  tabular  crystals  of  potassium  fluoborate  separate  out. 
When  these  are  recrystallized  from  hot  water  they  develop  prisms  with 
domes  and  pyramids.  The  indi  ces  of  refraction  are  lower  than  that  of  Canada 
balsam  and  the  double  refraction  is  so  low  that  it  can  only  be  recognized 
in  thick  crystals  with  a  gypsum  test  plate.  The  principal  zone  is  negative. 

Carbon. — If  carbon  is  present  in  a  rock  as  such  or  as  a  constit- 
uent of  an  organic  compound,  it  is  converted  into  a  carbonate  by 
fusing  the  substance  with  potassium  antimonate.  The  carbonate 
can  then  be  tested  by  its  effervescence  with  acid.  If  only  a  very 
small  quantity  of  carbonate  is  present,  the  rock  powder  is  spread 
out  with  water  on  an  object  glass  and  a  cover-glass  is  placed  over 
it.  Then  a  drop  of  dilute  hydrochloric  acid  is  placed  in  contact 
with  the  edge  of  the  cover-glass  and  a  piece  of  filter  paper  is 
inserted  under  the  other  side  of  the  glass.  The  acid  is  slowly 
drawn  under  the  cover  and  the  formation  of  small  gas  bubbles  in 
the  solution  can  be  observed  under  the  microscope. 

Silicon. — The  formation  of  skeletons  of  silica  in  a  bead  of 
microcosmic  salt  cannot  always  be  definitely  observed.  The 
best  method  for  the  detection  of  silica  is  a  micro-chemical  test 


170  PETROGRAPHIC  METHODS 

carried  out  in  the  same  manner  as  was  described  for  boron,  except 
that  sodium  chloride  is  used  instead  of  potassium  chloride.  The 
powder  may  also  be  decomposed  by  fusion  with  sodium  carbonate 
and  the  melted  mass  treated  with  pure  hydrofluoric  acid.  In 
both  cases  the  crystallization  of  sodium  fluosilicate  is  to  be 
observed. 

Titanium. — Minerals  in  which,  titanium  is  suspected  are  fused 
with  potassium  bisulphate,  dissolved  in  water  and  treated  with 
hydrogen  peroxide.  With  small  traces  of  titanium  the  solution 
is  colored  light  yellow,  and  with  larger  quantities  deep  brown. 
The  color  is  most  easily  recognized  if  the  operation  is  carried  out 
on  a  porcelain  plate. 

Zirconium. — It  is  difficult  to  prove  the  presence  of  zirconium 
oxide  in  rocks  with  absolute  certainty,  because  it  is  usually 
present  only  in  very  small  quantities.  The  substance  is  fused 
with  about  double  its  amount  of  sodium  carbonate  for  a  long 
time  and  the  fusion  product  dissolved  in  hot  acidified  water. 
Zirconium  is  found  in  the  insoluble  residue  as  small  six-sided 
crystals  similar  to  tridymite.  The  index  of  refraction  is  lower 
than  that  of  Canada  Balsam.  They  often  gather  into  packets 
similar  to  a  pile  of  coins.  Another  method  of  testing  for  zircon- 
ium consists  in  fusing  the  powder  with  potassium  bisulphate  and 
precipitating  the  zirconium  oxide  with  ammonia.  After  filtering, 
the  precipitate  is  dissolved  in  hydrochloric  acid  and  a  little  tin 
foil  is  dropped  into  the  solution.  If  zirconium  oxide  is  present 
the  solution  colors  turmeric  paper  brown,  while  solutions  contain- 
ing titanium  do  not  color  it  after  reduction. 

Tin. — If  a  mineral  is  thought  to  contain  tin,  it  is  fused  on  a  charcoal 
tablet  with  sodium  carbonate  and  potassium  cyanide.  The  tin  is  reduced  to 
a  metallic  bead  that  can  be  seen  with  a  lens.  If  the  bead  is  not  observed, 
the  melt  is  placed  on  a  platinum  foil  and  dilute  hydrochloric  acid  with  a 
very  little  platinum  chloride  added.  An  intense  red-brown  color  results  if 
tin  is  present.  A  borax  bead,  containing  a  little  copper  oxide  and  tin, 
heated  alternately  in  an  oxidizing  and  reducing  flame,  becomes  ruby 
colored.  This  is  a  very  sensitive  reaction. 

.  Vanadium. — Vanadium  is  often  present  in  rocks  in  very  small  quantities. 
The  rock  powder  is  fused  with  sodium  carbonate  and  saltpeter  and  the  melt 
dissolved  in  slightly  acidified  water.  The  solution  becomes  red  in  color 
when  hydrogen  peroxide  is  added,  provided  vanadium  is  present.  When  a 
grain  of  sal  ammoniac  is  added  to  a  solution  of  the  melt,  microscopic  crystals 
of  ammonium  metavanadinate  are  formed.  They  have  a  very  character- 
istic shape  like  that  of  a  whetstone  or  a  hatchet. 

Niobium  and  Tantalum. — A  mineral  is  fused  with  a  large  amount  of 


METHODS  OF  INVESTIGATION  171 

caustic  potash  to  test  for  niobates  and  tantalates.  The  melt  is  dissolved  in 
water  and  hydrochloric  acid  added.  Both  salts  are  thrown  down  as  a 
white  precipitate,  and  after  filtering,  the  precipitate  is  dissolved  in  sulphuric 
acid.  A  piece  of  zinc  is  dropped  into  the  solution  which  is  then  heated  to 
boiling.  Niobates  color  the  solution  smalt  blue,  the  color  being  retained 
for  a  long  time  upon  dilution  until  gradually  the  white  powder  forms  again. 
Tantalates  produce  a  grayish-blue  color  which  rapidly  fades  when  the 
solution  is  diluted. 

Phosphorous. — Phosphoric  acid  is  present  in  rocks  only  in 
small  amounts.  When  a  rock  powder  is  heated  to  redness  with 
magnesium  powder  in  a  glass  tube,  the  phosphoric  acid  is  reduced, 
causing  a  brilliant  pyrotechnic  display.  When  the  product 
formed  is  thrown  into  water  the  odor  of  phosphine  can  be  readily 
recognized  even  though  only  a  small  trace  of  phosphorous  is 
present.  By  another  method  the  rock  powder  is  digested  with 
nitric  acid  and  the  solution  treated  with  ammonium  molybdate. 
After  a  long  time  a  sulphur  yellow  precipitate  of  ammonium 
phosphomolybdate  is  formed.  Under  the  microscope  it  seems  to 
consist  of  small,  yellow,  rounded  dodecahedrons  or  of  delicate 
skeletal  crystals.  However,  if  soluble  silicates  are  present  at  the 
same  time,  there  is  danger  of  confusing  the  ammonium  phospho- 
molybdate with  ammonium  silicon  molybdate  which  may  be 
formed.  It  is  well  to  render  the  silica  insoluble  by  evaporating 
and  heating  on  the  object  glass,  and  then  to  redissolve  the  powder 
before  the  addition  of  ammonium  molybdate.  The  formation 
of  magnesium  ammonium  phosphate,  mentioned  above  un- 
der magnesium,  is  also  a  useful  reaction  for  the  detection  of 
phosphorous. 

Sulphur. — Sulphur  in  any  form  can  be  tested  by  fusing  a 
mineral  containing  it  with  sodium  carbonate  on  charcoal.  The 
melt  is  crushed  on  a  silver  coin  and  moistened.  A  brownish- 
black  spot  is  produced — Hepar  test.  The  melt  also  colors  an 
alkaline  solution  of  sodium  nitrocyanide  violet. 

Chlorine. — Chlorine  can  be  detected  when  a  mineral  powder 
is  fused  in  a  bead  of  microcosmic  salt  saturated  with  copper  oxide. 
The  flame  becomes  distinctly  blue.  The  micro-chemical  test  is 
also  good.  Silver  nitrate  is  added  to  an  ammoniacal  solution  of 
a  chloride  and  is  left  in  the  dark  to  evaporate.  The  resulting 
octahedrons  of  silver  chloride  have  a  high  index  of  refraction  and 
become  clouded  and  violet  when  left  in  the  light. 

Fluorine. — To  test  for  fluorine,  the  mineral  is  fused  with  salt 
of  phosphorous  in  a  small  glass  tube.  The  tube  is  etched  near 


172  PETROGRAPHIC  METHODS 

the  melted  material.  The  mineral  can  also  be  heated  with  con- 
centrated sulphuric  acid  in  a  platinum  crucible  on  the  cover  of 
which  a  drop  of  dialyzed  silica  adheres.  This  drop  is  later  re- 
moved and  placed  on  an  object  glass  covered  with  Canada  balsam 
and  some  sodium  salt  is  added.  The  characteristic  crystals  of 
sodium  fluosilicate  are  formed  upon  evaporation  if  fluorine  were' 
present. 

Finally,  the  test  for  water  must  be  mentioned.  Attention 
must  be  called  to  the  fact  that  with  many  minerals,  very  careful 
and  extended  drying  is  necessary  to  remove  all  the  hygroscopic 
water.  The  best  test  for  water  is  carried  out  in  the  following 
manner.  A  narrow  glass  tube  about  10  cm.  long  is  drawn  out  at 
one  end  into  a  capillary  and  dry  air  is  drawn  through  it  while 
heating.  Then  the  capillary  is  closed  by  fusion  and  the  rock 
powder  introduced  into  the  other  end  of  the  tube  which  is  like- 
wise closed.  During  this  process  care  must  be  taken  that  no 
moisture  is  introduced  into  the  tube  from  the  burner  gas.  Upon 
heating  the  powder  in  the  sealed  tube,  whatever  water  is  present 
can  be  driven  into  the  capillary  containing  a  small  quantity  of 
some  dry  dye.  The  dye  is  dissolved  by  the  drop  of  water  as  it 
condenses  and  imparts  an  intense  color  to  the  drop. 

b.  Special  Reactions 

Special  reactions  can  be  applied  either  to  isolated  grains  or  directly  to 
thin  sections.  If  the  slide  is  to  be  subjected  to  chemical  investigation  it 
is  left  uncovered,  or  if  covered,  the  cover-glass  can  be  removed  by  warming 
it  slightly.  The  section  itself  must  then  be  thoroughly  cleaned  from 
Canada  balsam  by  washing  with  benzol.  For  certain  reactions,  such  as 
heating  tests,  etc.,  the  rock  section  must  also  be  removed  from  the  object 
glass.  This  can  be  accomplished  by  heating  gently  to  melt  the  Canada 
balsam,  after  which  the  section  is  washed  for  some  time  in  benzol.  In 
most  cases  the  whole  section  is  not  subjected  to  chemical  tests  at  one  time, 
but  only  a  portion  of  it  is  used,  reserving  the  rest  of  it  for  other  reactions. 
The  cover-glass  is  cut  between  the  two  portions  with  a  diamond  cutter  and, 
if  the  section  also  is  to  be  removed  from  the  object  glass,  it  is  cut  through 
at  the  same  place.  In  some  cases  the  cover-glass  is  removed  and  the 
section  covered  with  liquid  Canada  balsam.  Then  the  particle  to  be  iso- 
lated is  focused  under  the  microscope  and  a  perforated  cover-glass  is  placed 
over  it  so  that  the  hole  in  the  glass  falls  directly  over  the  grain  to  be  studied. 
If  a  reaction  with  hydrofluoric  acid  is  to  be  made  the  cover-glass  can  be 
replaced  by  a  platinum  foil.  The  section  must  be  left  for  a  long  time  to 
dry  and  then  the  balsam  must  be  thoroughly  washed  out  of  the  small 
opening  over  the  mineral  grain.  It  is  then  ready  for  the  reaction. 


METHODS  OF  INVESTIGATION  173 

Certain  chemical  reactions  serve  to  clarify  rock  sections  because 
they  dissolve  out  those  constituents  which  make  it  clouded  or 
nontransparent.  Such  substances  are  chlorite,  zeolites  and 
other  scaly  decomposition  products,  or  iron  oxide  which  is 
present  as  a  finely  divided  pigment.  They  c'an  be  removed  by 
separating  the  section  from  the  object  glass  and  digesting  it  for 
a  long  period  with  dilute  hydrochloric  acid.  If  the  section  is 
impregnated  with  graphite  or  carbonaceous  material,  it  is  clari- 
fied by  roasting  on  a  platinum  foil.  A  wet  method  for  removing 
carbonaceous  material  is  most  favorable  under  certain  conditions, 
e.g.,  in  the  study  of  the  organic  structure  of  coals.  The  section  is 
treated  for  a  long  period  with  chlorous  acid,  page  150,  which 
dissolves  the  carbonaceous  matter.  Graphite  cannot  be  removed 
in  this  way. 

The  observation  of  etch  figures  has  often  been  suggested  as  a  means  of 
investigating  thin  sections,  but  no  reliable  results  can  be  obtained  in  this 
way.  Etch  figures  may,  however,  be  useful  to  distinguish  between  amor- 
phous substances  and  those  which  crystallize  in  the  cubic  system  because  the 
latter  would  give  regularly  bounded  figures. 

Staining  Methods. — The  staining  methods,  which  play  so 
important  a  role  in  organic  microscopy,  are  only  used  occasionally 
in  the  investigation  of  thin  sections  because  only  a  few  minerals 
in  their  natural  condition  would  absorb  the  stains  and  those  that 
do  are  principally  thick,  scaly  to  fibrous  aggregates  of  good 
cleavable  minerals,  but  even  they  can  scarcely  be  differentiated 
or  recognized  in  this  way.  Usually,  before  a  mineral  in  a  thin 
section  is  stained,  it  is  treated  on  the  object  glass  with  dilute 
acid  until  it  has  been  so  altered  that  it  is  covered  with  a  gelatinous 
precipitate.  This  gelatinous  material  absorbs  the  stain  evenly  and 
retains  it  so  that  it  cannot  be  washed  out.  After  the  etching 
action,  which  is  always  only  superficial,  the  acid  must  be  thor- 
oughly washed  away  before  the  stain  is  applied  or  it  may  be 
destroyed.  The  section  is  placed  in  an  evaporating  dish  filled 
with  distilled  water  to  which  a  few  drops  of  a  solution  of  Congo 
red  or  malachite  green  or  any  other  permanent  dye  have  been 
added.  If  it  is  left  to  stand  for  several  hours,  the  dye  is  absorbed 
quite  evenly  by  the  gelatinous  precipitate.  It  is  then  thoroughly 
washed  in  running  water,  dried,  and  covered  again  with  Canada 
balsam. 

Certain  silicates  dissolve  in  hydrochloric  acid  and  the  silica 


174  PETROGRAPHIC  METHODS 

separates  in  a  gelatinous  condition,  covering  the  thin  section. 
A  thin  film  of  dilute  hydrochloric  acid  is  put  on  the  mineral  in 
the  thin  section  so  that  the  etching  will  take  place  only  on  the 
surface  and  the  whole  mineral  will  not  be  dissolved.  This  reac- 
tion is  characteristic  for  nepheline,  which  is  otherwise  often 
very  difficult  to  recognize  in  the  ground  mass  of  a  basaltic  rock. 
But  plagioclase  and  scapolite  rich  in  lime,  sodalite,  melilite, 
wollastonite,  chlorite,  serpentine,  zeolites,  and  certain  rock 
glasses  give  the  same  reaction,  so  that  the  conclusion  that 
nepheline  is  present  is  only  warranted  if,  after  it  has  been 
colored,  the  mineral  shows  the  characteristic  cross  section  of 
nepheline. 

Aluminium  fluosilicate,  resulting  by  etching  aluminium  sili- 
cates with  hydrofluoric  acid,  is  also  a  gelatinous  substance  well 
adapted  to  such  color  tests.  It  is  especially  serviceable  for 
differentiating  between  feldspar  and  quartz  in  very  fine  grained 
aggregates.  Hydrofluoric  acid  is  allowed  to  act  for  a  short  time 
on  the  section  and  then  it  is  entirely  removed  by  heating  on  a 
water  bath.  The  stain  is  absorbed  by  the  gelatin  formed  on  the 
feldspar  while  the  quartz  is  not  colored  at  all.  In  this  case,  it  is 
best  to  use  dilute  alcohol  to  wash  the  section.  Other  silicates  of 
aluminium  that  are  attacked  by  hydrofluoric  acid,  e.g.,  cordierite, 
can  be  distinguished  from  quartz  or  from  silicates  containing  no 
aluminium  in  this  manner.  By  careful  operation  the  feldspars 
can  be  differentiated  by  these  tests  because  orthoclase  is  more 
difficultly  attacked  than  the  plagioclases  and,  among  the  latter, 
those  richest  in  lime  are  the  most  easily  attacked. 

Haiiyne,  containing  calcium  sulphate,  can  be  determined  by 
the  gypsum  crystals  formed  on  its  surface  when  it  is  treated  with 
dilute  hydrochloric  acid  and  dried.  Characteristic  hopper- 
shaped  growths  of  salt  also  appear  with  the  gypsum.  The  salt 
crystals  are  seen  alone  under  the  same  conditions  on  sodalite, 
noselite  and  nepheline.  A  characteristic  reaction  has  been 
found  for  certain  minerals  in  a  modification  of  this  test.  If, 
e.g.,  an  aluminium  silicate  containing  chlorine  (sodalite  and 
scapolite)  or  sulphur  (lazurite)  is  treated  with  a  solution  of  silver 
nitrate  in  dilute  hydrofluoric  acid,  the  gelatin  is  impregnated 
in  the  first  case  with  silver  chloride,  which  becomes  brown  when 
treated  with  a  photographic  developer,  and  in  the  second  case 
with  black  silver  sulphide.  In  this  manner  these  elements  that 
are  not  common  in  silicates,  can  be  detected. 


METHODS  OF  INVESTIGATION  175 

Precipitates  on  Thin  Sections. — A  dilute  nitric  acid  solution  of 
silver  nitrate  can  be  used  to  test  for  chlorine  in  such  minerals  as 
apatite.  Apatite  can  also  be  determined  by  a  localized  lemon- 
yellow  precipitate  which  tends  to  form  in  a  wreath  around  the 
apatite  grain  when  it  is  treated  with  a  concentrated  nitric  acid 
solution  of  ammonium  molybdate.  Heating  should  be  avoided  in 
this  reaction  because  then  the  silica  is  easily  dissolved  out  of  the 
surrounding  minerals  and  gives  rise  to  a  very  similar  precipitate 
of  ammonium  silicon  molybdate. , 

There  are  numerous  precipitation  reactions  by  means  of  which 
the  various  carbonates,  which  cannot  be  distinguished  by  their 
optical  properties,  can  be  differentiated.  Linck's  solution  is  a  solu- 
tion of  ammonium  phosphate  in  dilute  acetic  acid.  If  a  thin  sec- 
tion containing  carbonates  is  treated  with  it,  calcite  will  be 
entirely  dissolved  out,  while  dolomite  is  slowly  covered  with 
crystals  of  magnesium  ammonium  phosphate  arranged  in  charac- 
teristic aggregates.  A  dilute  solution  of  copper  sulphate  be- 
comes brilliant  blue  upon  the  addition  of  calcite,  but  with  dolo- 
mite it  remains  unchanged.  Calcite  rapidly  precipitates 
colored  gelatinous  alumina  from  a  solution  of  alum  or  neutral 
aluminium  sulphate,  containing  also  brilliant  green  or  fuchsine. 
A  coating  of  ferric  hydroxide  forms  on  calcite  in  solutions  of 
ferric  salts  and  this  coating  can  eventually  be  colored  black  by 
ammonium  sulphide.  Dolomite  remains  completely  unaltered 
for^a  long  time  in  both  of  the  above  cases,  but  it  gives  the  same 
reactions  upon  heating.  Calcite  cannot  be  distinguished  from 
aragonite  by  any  of  the  above  reactions,  but  the  tests  by  which 
they  can  be  differentiated  are  very  interesting.  If  the  minerals 
are  finely  powdered  and  boiled  with  a  dilute  solution  of  cobalt 
nitrate,  calcite  remains  perfectly  colorless  while  aragonite 
becomes  bright  violet.  If  boiled  long,  ten  minutes  or  so,  calcite 
is  colored  slightly,  but  in  the  meantime  aragonite  has  turned  dark 
violet.  A  solution  o£  ferrous  ammonium  sulphate  can  be  used 
to  distinguish  between  calcite  and  aragonite,  and  this  reaction 
can  be  carried  out  on  a  thin  section.  If  a  drop  of  the  cold  solu- 
tion is  placed  on  the  slide,  calcite  becomes  rust  colored,  while 
aragonite  turns  dark  blackish-green. 

Metallic  iron  occurs  now  and  then  as  a  constituent  of  rocks. 
It  can  be  detected  by  a  copper  sulphate  solution  in  which  it 
becomes  coated  with  metallic  copper,  or  by  Klein's  solution  which 
turns  deep  blue. 


176  PETROGRAPHIC  METHODS 

It  is  quite  evident  that  all  these  precipitation  reactions  can  be 
carried  out  on  single  isolated  grains.  This  is  of  great  importance 
in  the  case  of  such  minerals  as  scapolite,  which  is  present  only  in 
very  small  quantities  and  is  very  difficult  to  recognize. 

Alteration  by  Calcination. — Attempts  are  frequently  made  to 
produce  characteristic  changes  in  minerals  by  calcining  the 
section.  Thus  zeolites,  brucite,  hydromagnesite  and  cancrinite 
become  cloudy  when  heated  a  little  and  their  optical  properties 
are  somewhat  changed.  If  the  slide  is  heated  to  redness  and 
afterward  treated  with  silver  nitrate,  brucite  and  hydromagne- 
site will  be  coated  with  a  dark  brown  film  of  silver  hydroxide. 
Other  hydrous  minerals  such  as  chlorite,  serpentine,  etc.,  must 
be  calcined  at  very  high  temperatures  for  some  time  before  they 
become  cloudy.  They  turn  brown  first  and  at  higher  tempera- 
tures black,  due  to  the  oxidation  of  iron. 

Numerous  minerals  containing  iron  change  their  color  when 
moderately  heated  in  an  oxidizing  flame.  Thus,  olivine  becomes 
brownish-red  and  quite  pleochroic;  light-colored '  hornblende 
assumes  the  color  and  absorption  of  basaltic  hornblende.  Chlorite 
changes  in  a  similar  manner.  Epidote  and  orthorhombic 
pyroxenes  become  darker  when  heated  very  slightly,  while 
the  monoclinic  pyroxenes  free  from  sodium  darken  only  at  very 
high  temperatures,  and  those  rich  in  sodium  and  iron  melt  easily 
to  a  black  glass.  Some  varieties  of  colorless  cordierite  can  be 
made  blue  and  pleochroic  by  slow  and  careful  heating,  but  they 
lose  their  color  again  if  heated  higher.  Many  members  of  the 
sodalite  group  are  colored  an  intense  blue  by  heating  in  air 
while,  with  other  members,  this  change  takes  place  only  when 
heated  in  sulphur  vapor.  Finally,  the  calcination  test  can  be 
used  to  detect  aluminium  in  a  thin  section.  It  is  heated  with 
cobalt  solution,  then  washed  with  dilute  acid,  and  the  blue  color 
is  observed  in  reflected  light.  This  serves  to  distinguish  talc 
from  sericite. 

The  calcination  process  can  be  used  to  distinguish  between 
carbonaceous  substances  or  graphite  and  ores  in  a  rock  section. 
It  is  always  well  to  treat  the  section  with  hydrochloric  or  nitric 
acid  before  and  after  heating,  because  the  organic  substance  is 
very  frequently  intergrown  with  iron  ore.  Graphite  and  carbo- 
naceous material  both  burn  off  more  easily  if  they  are  in  a  finely 
divided  state. 


METHODS  OF  INVESTIGATION  177 

2.  Physical  Methods  of  Investigation 

Determination  of  the  Specific  Gravity. — Specific  gravity  is  the 
most  important  physical  property,  aside  from  the  optical  proper- 
ties, that  can  be  used  to  determine  a  mineral.  The  different 
methods,  which  the  physicists  employ  to  determine  it,  require 
for  the  most  part,  too  much  time.  The  accuracy  of  measurement 
with  a  hydrostatic  balance  or  a  pycnometer  is  not  at  all  commen- 
surate with  the  small  degree  of  purity  generally  possessed  by 
the  rock  minerals.  Further,  it  is  necessary,  for  such  accurate 
determinations  of  specific  gravity,  to  have  a  considerable  amount 
of  pure  material  available,  which  is  often  impossible.  We  must 
be  contented  with  less  accurate  methods,  but  this  has  the 
advantage  that  the  results  can  be  obtained  even  with  a  very 
small  grain  of  a  mineral  and  the  measurement  possesses  a  sufficient 
degree  of  accuracy  for  our  purpose. 

The  method  generally  employed  in  petrography  to  determine 
the  specific  gravity  consists  in  putting  a  small  splinter  of  the 
mineral  in.  question  into  one  of  the  heavy  liquids  described  on 
pages  156  to  159.  The  liquid  is  gradually  diluted  until  that 
point  is  reached  at  which  liquid  and  grain  have  the  same  specific 
gravity,  so  that  the  grain  remains  suspended  in  any  part  of  the 
liquid  when  it  is  agitated  and  is  not  driven  up  or  down.  If  a 
crystal,  which  is  apparently  homogeneous,  is  broken  into  frag- 
ments, it  will  be  found  impossible  to  bring  all  these  pieces  into 
a  state  of  suspension  at  the  same  time,  but  a  medium  grain  will 
be  suspended,  while  some  will  sink  slowly  and  others  will  rise. 
These  various  fragments  of  one  and  the  same  crystal  appear  to 
have  different  densities.  The  phenomenon  depends  upon  the 
fact  that  there  are  minute  inclusions,  cleavage  cracks,  etc.,  in  the 
mineral  and  these  are  not  constant  throughout  the  crystal,  but 
are  different  in  the  different  fragments. 

Cleavage  cracks,  gas  bubbles,  and  liquid  inclusions  lower  the 
specific  gravity  of  a  mineral,  while  inclusions  of  heavy  minerals 
raise  it.  In  general  then,  the  specific  gravity  is  most  approxi- 
mately obtained  in  a  solution  in  which  part  of  the  grains  float, 
while  the  others  sink.  The  density  of  the  liquid  is  then  obtained 
by  means  of  an  aerometer,  the  best  form  of  which  is  the  Westphal 
balance,  Fig.  177. 

The  two  arms  of  the  balance  beam  are  in  equilibrium.  The 
pointer  on  the  arm  J  plays  exactly  on  the  point  opposite  it  when 
12 


178 


PETROGRAPHIC  METHODS 


/  \ 


FIG.  177. — Westphal  Balance. 


the  hydrometer,  usually  prepared  in  the  form  of  a  thermometer, 
is  suspended  from  E  by  a  fine  platinum  wire.  If  the  thermome- 
ter is  immersed  in  water  at  4°  C.,  it  is  evident  that  it  will  be  buoyed 
up  and  the  balance  will  be  thrown  out  of  equilibrium.  The 
buoyant  force  is  equal  to  the  weight  of  the  volume  of  water  at 
4°  C.,  displaced  by  the  thermometer  when  it  is  immersed.  If  a 
weight  m,  equal  to  the  weight  of  this  volume  of  water,  is  suspended 

with  the  thermometer  at  E,  the 
balance  will  again  return  to  a  state 
of  equilibrium. 

The  balance  is  supplied  with  four 
different  weights  in  the  form  of 
riders  and  these  are  calibrated  ac- 
cording to  the  volume  of  water  at 
4°  C.  displaced  by  the  hydrometer. 
They  correspond  to  0.01,  0.1,  one 
and  three  times  the  weight  of  it. 
In  addition  the  balance  beam  is 
divided  into  ten  equal  parts  so  that 
the  specific  gravity  of  a  liquid  can  be  measured  accurately  to  the 
third  decimal  place  and  can  be  estimated'  to  the  fourth.  Thus, 
it  can  be  measured  far  more  accurately  than  is  necessary  when 
the  impurity  of  the  mineral  is  considered. 

The  specific  gravity  of  the  heavy  liquids  that  are  employed  changes  quite 
rapidly  with  a  change  in  temperature,  so  the  whole  determination  must  be 
carried  out  at  a  temperature  as  nearly  constant  as  possible.  If  this  is  not 
possible,  the  temperature,  at  which  the  mineral  fragments  were  brought  into 
suspension  and  that  at  which  the  specific  gravity  of  the  liquid  was  deter- 
mined, must  be  taken.  The  error  introduced  by  the  change  of  temperature 
can  be  eliminated  by  considering  the  coefficient  of  expansion  of  the  liquid. 
It  is  quite  useless  to  make  an  extra  effort  to  work  at  constant  temperatures 
because  the  temperature  variations  ordinarily  experienced  do  not  affect  the 
specific  gravity  of  a  mineral  and  only  the  changes  in  the  liquid  need  to  be 
considered.  The  expansion  and  contraction  of  a  heavy  liquid  can  be 
demonstrated  best  by  bringing  a  crystal  into  suspension  in  a  heated  room 
and  then  placing  the  vessel  in  a  cool  room.  After  a  short  time  the  crystal 
rises  again  and  floats.  If  the  vessel  is  placed  near  a  stove  the  crystal  will 
be  seen  to  sink  rapidly.  It  is  also  to  be  noted  that  a  crystal  fragment  turns 
upon  its  sharp  edge  when  the  specific  gravity  of  it  and  of  the  liquid  are  about 
the  same.  This  is  an  indication  that  further  dilution  must  be  continued 
extremely  slowly  and  carefully.  If  the  proper  point  is  exceeded  and  the 
crystal  begins  to  sink,  the  density  must  be  increased  again  by  the  addition 
of  the  concentrated  liquid. 

It  is  not  necessary  at  this  time  to  consider  any  apparatus  for  determining 


METHODS  OF  INVESTIGATION  179 

the  specific  gravity  of  soluble  substances  because  petrographers  rarely  have 
to  deal  with  such  minerals.  Furthermore,  the  solvent  effect  of  the  organic 
liquids,  which  are  used,  and  of  the  solutions  of  inorganic  salts  is  extraordinar- 
ily variable.  If  one  group  of  liquids  cannot  be  used  because  of  its  solvent 
action  on  a  mineral,  another  can  be  employed  in  almost  all  cases.  On  the 
other  hand,  minerals  with  a  higher  density  than  the  heaviest  liquid  obtain- 
able are  not  rare.  The  method  of  procedure  then  is  to  knead  a  mass  of 
wax  and  minium  thoroughly  in  such  proportions  that  the  specific  gravity 
shall  be  about  2.5.  The  mineral  is  pressed  into  a  weighed  amount  of  this 
mass  called  the  float.  The  amount  of  the  float  is  so  chosen  that  the  specific 
gravity  of  the  combination  amounts  to  about  3.0.  If  g  equals  the  weight 
of  the  float  and  p  its  specific  gravity,  gf  the  weight  of  the  mineral  and  P  the 
specific  gravity  of  the  combination,  the  specific  gravity  of  the  mineral  p/ 

g'P 

can  be  calculated  from  the  formula  p' = —  — .        Fine  feathery 

g+g'  -Pg/p 

tongs  of  thin  glass  threads  can  be  made  and  used  as  floats.  The  mineral 
grain  is  fastened  in  the  tongs.  In  either  case  it  is  absolutely  necessary  not 
to  use  too  small  a  fragment  of  the  mineral  because  in  that  case  the  error 
introduced  becomes  too  large. 

Another  apparatus  for  determining  the  specific  gravity  of  liquids  is 
based  upon  the  law  that  the  heights  of  columns  of  liquids  retaining  equilib- 
rium in  communicating  tubes,  are  inversely  proportional  to  their  specific 
gravities.  If  the  liquid  to  be  determined  is  in  one  arm  of  the  tube  and 
water  in  the  other,  the  difference  in  height  of  the  columns  gives  the  specific 
gravity  of  the  liquid  directly.  This  apparatus  can  also  be  arranged  to 
make  separations  according  to  specific  gravity. 

Determination  of  Hardness,  Cleavage,  etc. — Now  and  then  it  is  advan- 
tageous to  determine  the  hardness  of  a  mineral.  However,  the  small  grains 
that  are  obtainable  are  too  small  to  make  a  hardness  test  by  hand.  A 
thin  bar  of  lead  is  planed  on  one  end.  An  indicator  of  known  hardness  is 
chosen  from  the  scale  of  hardness  and  one  surface  is  polished.  The  mineral 
grains,  whose  hardness  is  to  be  determined,  are  placed  on  this  surface. 
Then  the  lead  bar  is  pressed  against  it  firmly  and  moved  back  and  forth. 
The  grains  sink  into  the  soft  lead  and  are  held  firmly  as  if  in  a  setting  and 
are  moved  about  over  the  polished  surface.  If  the  grains  are  harder  than 
the  indicator,  fine  scratches  will  be  observed  on  the  surface  by. the  aid  of 
a  lens.  The  more  accurate  methods  used  in  physics  for  determining  the 
hardness  of  substances  are  of  no  consequence  in  the  case  of  rock-forming 
minerals.  It  may  be  mentioned  that  the  hardness  of  a  mineral  or  a  rock, 
i.e.,  its  resistance  to  being  scratched,  is  often  confused  with  its  resistance  to 
abrasion.  There  are  soft  minerals  that  can  be  but  slightly  abraded.  They 
are  tough  like  serpentine.  Then  there  are  others  that  are  very  hard,  but 
because  of  their  brittleness,  are  more  easily  worn  away  than  would  be 
expected  from  their  hardness.  .  Quartz  is  one  of  this  kind. 

The  determination  of  the  hardness  and  the  tendency  to  abrasion  of  a 
mineral  is  frequently  greatly  influenced  by  cleavage,  because,  parallel  to 
the  cleavage,  scratching  as  well  as  abrasion  encounters  less  resistance. 
Cleavage  is  nearly  always  quite  evident  in  a  thin  section  because  the 
minerals  are  shattered  more  or  less  along  their  cleavage  planes  during  the 


180  PETROGRAPHIC  METHODS 

process  of  grinding.  The  degree  of  cleavage  can  be  easily  estimated  in  a 
mineral  cross  section  by  the  number  of  the  cleavage  cracks,  and  whether 
they  are  long  and  straight  or  not.  Compare  Part  I,  p.  44.  The  cleavage 
form  and  the  cleavage  planes  can  be  determined  from  the  angle  which  the 
cracks  \iake  with  each  other  in  a  thin  section,  but  care  must  be  taken  to 
determine  the  cleavage  only  in  sections  perpendicular  to  the  cleavage 
planes.  Those  are  sections  in  which  the  cracks  do  not  shift  sideways  when 
the  focus  of  the  microscope  is  changed  from  the  upper  to  the  lower  surface 
of  the  mineral.  An  example  of  this  is  the  distinction  of  pyroxene,  with  a 
prismatic  cleavage  of  approximately  90°,  from  amphibole  with  a  prismatic 
cleavage  of  about  124°.  A  section  of  pyroxene  cut  obliquely  to  the  cleavage 
can  show  exactly  the  cleavage  angle  of  hornblende  and  vice  versa. 

The  production  of  a  percussion  figure  is  of  special  interest  in  the  investi- 
gation of  micaceous  minerals.  A  minute  scale  of  the  mineral  is  placed  on 
a  smooth  cork  and  a  vertical  needle  is  arranged  in  a  support  over  it  so 
that  it  can  move  up  and  down  freely.  The  needle  is  struck  a  sharp  blow 
with  a  watch  spring  and  driven  into  the  plate.  A  series  of  hexagons  can 
be  observed  around  the  hole  thus  produced.  See  the  mica  group,  p.  302. 
The  angles  of  the  hexagons  are  bisected  by  quite  straight  cracks  of  which 
two,  lying  diametrically  opposite  each  other,  are  more  pronounced  than  the 
others  and  are  called  the  principal  rays.  The  direction  of  these  principal 
rays  corresponds  to  the  plane  of  symmetry  of  mica. 

The  study  of  magnetism  is  confined,  for  the  most  part,  to  the  effect  of  a 
mineral  on  a  magnetic  needle  so  mounted  as  to  be  delicately  sensitive. 
Only  a  very  few  minerals  affect  it  naturally,  particularly  magnetite  and  pyr- 
rhotite.  All  minerals  rich  in  iron  will  affect  a  magnetic  needle  after  they  have 
been  fused.  Polar  magnetism  also  occurs  in  rocks,  but  the  distribution  of  the 
two  poles  is  usually  very  complicated  and  apparently  without  any  regularity. 
Conductivity  of  electricity  is  not  often  used  in  studying  rock  minerals,  but 
is  valuable  to  distinguish  between  graphite  and  coal.  Graphite,  fastened 
in  zinc,  becomes  coated  with  metallic  copper  in  a  copper  sulphate  solution. 


CHAPTER  X   . 
Development  of  Rock  Constituents 

External  Form. — The  outlines  of  the  rock-forming  minerals  are 
extremely  simple.  Well  developed  crystals,  the  outlines  of  which 
are  generally  not  very  sharp,  are  quite  rare  and,  when  they  do 
occur,  they  are  simple  and  do  not  show  combinations  of  many 
faces.  Rock  constituents  crystallographically  developed  in  this 
manner,  as  the  phenocrysts  of  the  porphyries,  Fig.  178,  or  the 
garnets  of  a  mica  schist,  are  termed  automorphic  (Greek  autos, 
self;  morphe,  form)  or  idiomorphic  (Greek  idios,  own).  A  partial 
crystallographic  development  is  observed  far  more  frequently. 
Feldspar  in  granite  shows  plainly  its  own  crystal  development 
next  to  the  quartz,  but  not  to  the  mica.  This  is  called  hypidio- 
morphic  development  (Greek  hypo,  almost) .  The  most  frequent 


FIG.  178. — Quartz  Porphyry,  Rochlitz,  Saxony. 

case  is  that  in  which  the  mineral  grains  lie  beside  each  other 
without  any  indication  of  crystallographic  form.  Such  a  gran- 
ular aggregate  is  said  to  be  xenimorphic  (Greek  xenos,  foreign)  or 
allotriomorphic  (Greek  allotrios,  foreign).  This  type  of  develop- 
ment is  seen  most  plainly  in  granular  limestones  and  may  be 
observed  macroscopically. 

Under  the  microscope  also,  these  three  types  of  development  can  be 
sharply  differentiated  from  each  other,  as  Figs.  179  to  181  show.     In  Fig. 

181 


182 


PETROGRAPHIC  METHODS 


180  the  development  of  the  quartz,  the  lightest  material  in  the  cut,  is  very 
distinctive.  It  fills  up  the  spaces  between  the  other  constituents  and  ob- 
tains its  form  entirely  from  them.  It  is  designated  as  interstitial  material 
or  mesostasis  (Greek  mesos,  middle;  stasis,  mass). 


FIG.  179.— Porphyric  Structure.     Granite 
Porphyry,  Ernsthofen,  Odenwald. 


FIG.    180. — Hypidiomorphic      Structure. 
Quartz  Diorite,  Schwarzenberg,  Vogesen. 


After  F.  Berwerth. 


Rocks  do  not  in  all  cases  consist  entirely  of  crystallized  constituents. 
Sometimes  amorphous  substances  make  up  part  of  the  rock.  This  is  some- 
times rock  glass  formed  from  the  magma,  occurring  either  as  a  subordinate 
base  or  as  a  prominent  constituent.  Sometimes  it  is  deposited  from  an 


FIG.  181. — Allotriomorphic    Structure. 
Quartzite,  Perosa,  Cottic  Alps. 


FIG.  182. — Sharp  crystals,  Uralite  and  Plagio- 
clase  in  Uralite  Porphyry,  Predazzo,  Tyrol. 


aqueous  solution  and  is  then  usually  opal.     Naturally  these  substances 
never  possess  characteristic  forms. 

It  may  be  said  that  the  smaller  the  amount  of  a  constituent  in  a  rock  and 
the  smaller  the  individuals,  the  more  perfect  is  the  crystallographic  develop- 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      183 

ment.  Those  minute  crystals  occurring  as  accessory  components  of  the 
eruptive  rocks,  like  zircon,  xenotime,  apatite,  monazite,  and  chrysoberyl, 
which  are  only  present  in  very  small  quantites  and  belong  to  the  oldest 
period  of  solidification  of  the  rock,  are  often  as  well  developed  as  models 
and  sometimes  show  an  abundance  of  faces.  Likewise  small  crystals  of 
feldspar,  pyroxene,  etc.,  which  have  separated  from  the  glassy  ground  mass 
of  an  eruptive  rock,  sometimes  have  quite  sharp  forms.  Larger  individuals 
are  not  so  apt  to  show  this,  either  because  they  did  not  have  a  good  develop- 
ment originally  or  because  they  have  later  been  partly  redissolved.  The 
smaller  crystals  are  not  so  liable  to  resorption. 

The  size  of  the  rock-forming  constituents  is  quite  variable. 
Especially  large  dimensions  are  observed  only  in  pegmatites, 
which  are  scarcely  to  be  considered  as  rocks  in  the  narrower  sense. 
Feldspar  crystals  are  found  in  them  with  edges  several  meters 
long  and  mica  plates  as  big  as  a  table.  In  rocks,  mineral  individ- 
uals as  large  as  a  head  or  fist  are  quite  scarce;  those  whose 
largest  dimension  is  one  or  more  centimeters  are  more  common. 
Even  these  are  more  apt  to  be  found  among  the  phenocrysts  of 
porphyric  rocks.  The  dimensions  are  usually  smaller  in  granular 
rocks  from  a  diameter  of  about  1  cm.  down  to  individuals  which 
cannot  be  seen  with  the  naked  eye.  The  granular  rocks  may  be 
medium  granular,  or  fine  granular,  or  may  appear  to  the  naked  eye 
to  be  aphanitic.  The  ground  mass  of  porphyric  rocks,  is  generally 
macroscopically  aphanitic  and  often  under  the  microscope  is 
composed  of  such  minute  individuals  that  a  very  thin  section 
and  strong  magnification  are  necessary  to  resolve  it.  The 
minutest  rock  constituents,  often  well  bounded  crystallograph- 
ically,  which  are  so  small  that  they  appear  in  the  slide  as  specks, 
are  called  microlites  (Greek  micros,  small) .  This  name  was  orig- 
inally given  to  a  rod-like  formation  which  was  afterward  called 
belonite  (Greek  belone,  arrow  heads) . 

The  crystallographic  habit  is  in  general  quite  characteristic 
for  any  one  species.  Apatite  develops  in  needles;  mica  is 
tabular;  epidote  is  elongated  parallel  to  the  b  axis;  hornblende 
parallel  to  the  c  axis;  and  many  feldspars  parallel  to  the  a  axis,  Fig. 
183,  p.  184.  In  thin  sections  the  cross  sections  have  a  character- 
istic form  and  a  distinguishing  principal  zone,  the  optical  char- 
acter of  which  aids  in  determining  the  mineral.  This  is  found  in 
column  Chz  in  the  table  at  the  end  of  the  book.  Acicular  or  thin 
tabular  minerals  give  mostly  lath-shaped  cross  sections.  Other 
minerals,  e.g.,  quartz  or  calcite  are  developed  more  nearly  iso- 
metrically.  Their  cross  sections  show  no  characteristic  elonga- 


184  PETROGRAPHIC  METHODS 

tion  and  they  have  no  principal  zones,  Fig.  184.  In  other  cases, 
e.g.,  corundum  and  topaz,  the  individuals  are  sometimes  elong- 
ated parallel  to  one  crystallographic  axis  and  sometimes  to 
another,  and  the  optical  character  of  the  principal  zone  varies 
accordingly. 

The  mineral  individuals  of  a  rock  are  frequently  simple  crystals.  If 
they  are  well  bounded  crystallographically,  the  form  of  the  crystal  can 
generally  be  inferred  from  the  outlines  of  several  sections  differently  ori- 
entated. However,  too  much  importance  should  not  be  given  to  the  ap- 
pearance of  one  or  more  cross  sections.  The  predominant  section  of  a 
mineral  developed  prismatic  or  tabular  is  in  both  cases  lath-shaped. 
The  laths  from  prismatic  crystals  have  a  width  about  equal  to  the  thick- 
ness of  the  crystal,  while  those  from  tabular  crystals  are  much  broader. 


FIG.  183.— Crystals  with  Principal  Zone         FIG.  184.— Crystals  without  Principal 
Grachyte,  Nolles.     (After  Berwerth.)  Zone.     Quartzite,  Steiermark. 

Such  minerals,  as  apatite,  often  give  six-sided  sections  which  have  no  effect 
on  polarized  light  and  even  in  convergent  polarized  light  cannot  be  readily 
distinguished  from  isotropic  minerals.  Again  in  a  great  number  of  rocks 
including  some  igneous  types,  tabular  or  acicular  crystals  of  uniaxial  min- 
erals are  all  arranged  parallel  so  that  they  extinguish  four  times  when 
rotated  between  crossed  nicols,  but  as  a  uniaxial  figure  cannot  be  obtained, 
these  are  often  mistaken  for  biaxial  minerals. 

Twinning. — Twinning  is  exceptionally  widespread  in  rock-forming 
minerals.  It  can  be  recognized  partly  in  the  form  of  the  section.  In 
cubic  twins  and  in  uniaxial  twins,  in  which  the  optic  axes  are  parallel, 
this  is  the  only  way  to  recognize  them.  If  reentrant  angles  cannot  be  seen, 
the  twinning  will  not  be  observed  in  rocks,  even  though  it  is  very  widespread, 
as  in  the  case  of  quartz.  Cruciform  twins,  Fig.  185,  and  juxtaposition  twins, 
Fig.  186,  show  readily  in  the  external  form.  In  penetration  twins  this  in- 
dication is  lacking  and  is  never  observed  where  the  rock  constituents  do 
not  have  an  external  form. 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      185 


Twins  of  uniaxial  crystals  with  their  optic  axes  oblique  to  each  other 
and  twins  of  biaxial  crystals  can  generally  be  recognized  in  polarized  light. 
In  the  first  case,  the  two  individuals  separated  by  a  sharp  plane  extinguish 
differently,  and  in  the  second  case  even  though  the  elasticity  axes  are 
parallel  the  two  parts  will  show  different  interference  colors.  This  is  par- 
ticularly notable  in  cyanite.  In  such  cases  as  epidote  where  the  twinning 
plane  is  very  nearly  parallel  to  the  bisectrix  of  an  optic  angle,  that  is,  almost 
90°,  the  twinning  may  not  be  apparent. 

Besides  simple  twinning,  repeated  twinning  or  lamination  is  as  common 
in  rock-forming  minerals  as  it  is  in  other  minerals.  It  is  rarely  observed 
in  cubic  minerals,  but  is  very  frequent  in  hexagonal  (calcite),  tetragonal 
(rutile),  orthorhombic  (cordierite),  monoclinic  (diallage),  and  triclinic 
minerals  (plagioclase) .  The  two  series  of  differently  orientated  individuals 
are  intergrown  with  each  other  in  a  more  or  less  regular  manner  in  the  form 


FIG.  185.— Penetra-     FIG.  186.— Mono-     FIG.  187.— Twinning       FIG.  188. — Lattice 
tion  Twin  with  Inclined        clinic  Twin.  Lamination.  Lamination. 

Axis.     Staurolite.  Diopside. 

Plagioclase. 

of  lamellae,  Fig.  187.  In  the  crystal  systems  of  higher  symmetry  several 
series  of  lamellae  may  cross,  producing  a  lattice  effect.  These  lamellae  are 
of  the  same  shape  and  the  twinning  is  parallel  to  the  different  faces  of  one 
form.  Such  is  the  case  with  calcite  twinned  parallel  to  — 1/2  R.  In  the  crystal 
systems  of  lower  symmetry,  this  lattice  structure  may  appear,  but  is  due  to 
the  presence  of  two  series  of  lamellae  twinned  according  to  different  laws. 
Albite,  Fig.  188,  is  a  good  example  of  this,  where  albite  and  peri-cline  twin- 
ning occur  together.  Twinning  lamination  may  be  caused  by  the  effect  of 
pressure  on  a  mineral.  This  can  be  demonstrated  with  calcite  which  pos- 
sesses the  property  of  gliding.  In  other  cases,  particularly  in  the  plagio- 
clases,  which  are  so  important  to  petrographers,  this  is  positively  not  the 
case  as  is  shown  in  the  schist  belt  of  the  Central  Alps.  Here  some  of  the 
rocks  have  been  greatly  deformed  locally,  but  the  plagioclases  show  almost 
no  lamination. 

Aggregates. — The  occurrence  of  several  individuals  of  a  min- 
eral together  is  called  an  aggregate.  Minerals,  which  have  no 
characteristic  principal  zone  generally  form  granular  aggregates, 
while  those  with  prismatic  or  acicular  development  tend  to  form 


186 


PETROGRAPHIC  METHODS 


columnar  to  fibrous  aggregates,  such  as  are  macroscopically 
apparent  in  sillimanite  intergrown  with  quartz.  Minerals 
possessing  a  tabular '  habit  develop  scaly  or  flaky  aggregates. 
Finely  divided  aggregates  of  colorless,  low  double  refracting 
minerals  appear  as  homogeneous  individuals  in  a  thin  section  in 
ordinary  light,  but  their  true  nature  becomes  apparent  in 
polarized  light,  if  they  are  not  isotropic  substances,  because  the 
various  parts  extinguish  differently — aggregate-polarization.  If 
the  aggregates  are  very  fine  an  even  illumination  will  be  observed 
between  crossed  nicols  with  low  magnification  and  this  will  not 


FIG.  189.— Liparite.     Glashuttental, 
Hungary. 


FIG.  190.— Oolite,  Vilstal,  Algan. 


be  affected  by  rotating  the  stage.  Only  with  high  power  ob- 
jectives is  it  possbile  to  resolve  such  aggregates  into  their 
constituent  parts.  This  phenomenon  is  quite  apparent  in  dense 
aggregates  of  sericite  or  talc. 

The  most  important  of  the  fibrous  or  scaly  aggregates  are  those 
in  which  the  various  individuals  are  arranged  in  a  radiating  man- 
ner around  a  point  or  a  line — spherulites  or  axiolites.  Such 
structures  are  characteristically  developed  in  eruptive  rocks 
that  are  rich  in  silica  and  have  solidified  rapidly.  The  spheru- 
lites, Fig.  189,  correspond  in  general  to  a  eutectic  mixture  of 
quartz  and  feldspar.  Analogous  structures  are  by  no  means 
rare  in  secondary  depositions  in  cracks  of  the  rocks,  e.g.,  in 
chalcedony  or  aragonite.  They  constitute  the  formation  known 
as  oolite,  Fig.  190.  In  organic  nature  the  spherulitic  develop- 
ment is  .sometimes  quite  marked,  e.g.,  in  the  skeletal  portions 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      187 

of  foraminifera.  In  these,  axiolites  predominate,  e.g.,  in  the 
cross  sections  of  nummulites.  Frequently  the  radial  structure 
is  lost  and  a  granular  aggregate  is  developed  in  its  place  by 


FIG.  191. — Granospherite.     Recrys- 
talized  Oolite. 


FIG.  192. — Marble  (Ordinario), 
Carrara,  Italy. 


recrystallization.  The  outer  form  and  sometimes  also  the 
concentric  structure  may  be  preserved  during  this  process.  It 
gives  rise  to  a  granospherite,  Fig.  191.  Distinctions  can 
be  made  in  the  character  of  the  granular  aggregates.  Both 


FIG.  193. — Marble  (Statuarid), 
Carrara,   Italy. 


FIG.    194.— Zonal  Plagioclase  in  Pitchstone. 
Cunardo  near  Lugano.     (After  E.  Cohen.) 


end  members  of  an  even  grained,  granular  rock  show  the  min- 
eral grains  simply  lying  beside  each  other — mosaic  structure, 
Fig.  192, — or  very  intimately  dovetailed  with  each  other, 
Fig.  193. 


188 


PETROGRAPHIC  METHODS 


Growth  and  Solution. — The  composition  of  a  magma  gradually 
changes  during  the  course  of  crystallization.  This  gives  rise  to 
zonal  development  in  isomorphous  minerals,  particularly  in  the 
plagioclases  and  pyroxenes,  Fig.  194,  in  which  twinning,  if  it  is 
present,  continues  unchanged  through  all 
zones.  In  pyroxenes  rich  in  sodium  and 
titanium  various  pyramids  are  so  grown 
together  as  to  produce  an  hourglass  structure, 
Fig.  195.  It  is  noteworthy  that  this  appear- 
ance, which  has  been  shown  to  be  dependent 


FIG.     195. — Hourglass 
Structure  in  Ryroxene. 


FIG.  196. — Skeletal  Crystals  of  Olivine. 


upon  a  gradual  change  of  the  chemical  composition  of  the  crys- 
tallizing mass,  occurs  but  exceptionally  in  contact  rocks  and,  if 
it  does  occur,  the  law  of  zonal  construction  may  be  just  reversed. 
In  eruptive  rocks  this  law  seems  to  be  constant,  e.g.,  in  the 
plagioclases  where  the  outer  zones  are  always  richer  in  silica. 

Various  phases  in  the  process 
of  the  formation  of  rocks  may 
be  recognized  from  the  develop- 
ment of  the  rock  constituents 
as  has  been  thoroughly  estab- 
lished in  the  Allgemeine  Ges- 
teinskunde  by  E.  Weinschenk. 
Only  the  external  forms  will 
be  described  here.  The  crys- 
tallization of  a  mineral  is  more 
complete  if  it  takes  place 
slowly  from  a  very  dilute  solu- 
tion. In  the  glassy  eruptive 

rOCks,  Which   have    COOled    SUd-      Fl°-  ^.-Skeletal  Crystals .in  Pitchstone. 
.       .          .  (From  Arran.) 

demy,    incomplete    crystalliza- 
tions are  frequently  observed.     The  growth  on  the  edges  and 
corners  is  most  noticeable  and  the  faces  remain  as  more  or  less 
deep    indentations — ruin-like    development.     Crystal    skeletons 
may  result,  which  are  hard  to  define  crystallographically,  but 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      189 

are  sometimes  quite  regular,  Fig.  196,  and  at  other  times  grow 
in  variously  shaped  structures  not  unlike  organic  forms.  The 
exact  nature  of  them  cannot  be  determined  in  most  cases,  Fig. 
197.  The  last  series  of  growth  forms  is  very  simple.  In 
obsidian,  crystallites,  Fig.  198,  which  cannot  be  determined,  are 
frequently  found.  They  may  appear  as  dark  points — globulites — 

a  h 


•':*!>* 

•>v  -::•.. 

~ ;  w 

FIG.  198.— Crystallites, 
a  Globulite,  b  Margarite,  c  Cumulite,  d  Baculite  and  e  and  f  Trichite. 

which  may  be  grouped  together  like  a  string  of  pearls — marga- 
rites — or  in  irregular  bunches — cumulites.  Sometimes  they  are 
dark  rods — baculites — or  curved  fine  hair-like  forms — trichites. 
These  often  form  star-shaped  aggregates,  Fig.  198,  /. 

Other  irregularities  in  the  form  of  the  constituents  of  eruptive 
rocks  are,  unlike  those  already  described,  of  a  secondary  nature, 
and  are  caused  by  the  sol- 
vent action  of  the  magma 
on  the  crystals  which  have 
formed  in  it.  This  is  natur- 
ally observed  most  com- 
monly in  porphyric  rocks  in 
which  the  physical  condi- 
tions have  been  intensely 
changed  during  the  process 
of  crystallization.  The  phe- 
nomenon, which  is  called 
magmatic  corrosion  or  re- 
sorption,  is  rarer  in  rocks 
rich  in  glass  than  in  rocks 

fe  .  FIG.  199.— Corroded  Quartz. 

poor  in  glass,  because  in  the 

latter  the  process  of  solidification  has  been  slower  and  more 
time  given  to  the  solution  process.  The  external  form  is  often 
considerably  changed.  The  corners  and  edges  are  rounded, 
and  deep  indentations  made  in  the  faces  until  finally  only 
an  irregular  fragment  of  the  crystal  is  left,  Fig.  199.  These 


190 


PETROGRAPHIC  METHODS 


phenomena  are  especially  pronounced  in  quartz,  olivine  and 
the  minerals  of  the  sodalite  group. 

Chemical  changes  also  result  under  these  conditions.  Min- 
erals of  the  amphibole  and  mica  groups  containing  hydroxyl 
suffer  molecular  rearrangement  in  which  opaque  iron  oxide 
separates  out  and  a  black  magmatic  border  is  produced.  This 
process  may  continue  until  finally  a  more  or  less  sharply  defined 
speck,  showing  aggregate-polarization,  is  left  in  place  of  the 
original  crystal.  They  are  very  difficult  to  determine,  but  in  the 
tuffs  of  such  eruptive  rocks,  which  preserve  the  material  in  the 
first  stages  of  its  occurrence,  the  unaltered  mineral  is  often  seen 
in  large  quantities. 

It  is  noteworthy  that  the  accessory  minerals  in  granular  lime 
stones,  especially  the  silicates,  often  show  surface  characteristics 
analogous  to  magmatic  resorption  so  that  they  appear  to  have 
been  partly  fused. 

Cleavage,  Parting,  and  Mechanical  Deformation. — The  signifi- 
cance of  cleavage  in  the  determination  of  rock-forming  minerals 
has  already  been  pointed  out.  The  more  complete  the  cleavage 
the  straighter  and  finer  do  the  cleavage  cracks  appear  in  micro- 
scopical preparations  and,  if  the  cleavage  is  perfect  in  colorless 
low  double  refracting  minerals  as  in  feldspar,  it  is  often  quite 
difficult,  even  when  the  condensing  system  of  lenses  is  lowered  as 


FIG.  200.— Perfect 


FIG.  201. — Distinct 


FIG.  202. — Imperfect. 


Cleavage. 

far  as  possible,  to  recognize  the  cleavage  cracks  distinctly.  It 
may,  however,  be  noted  that  they  often  become  visible  in  polar- 
ized light  especially  with  the  gypsum  test  plate.  If  the  cleavage 
is  less  perfect,  the  cleavage  cracks  are  not  so  straight,  do  not 
extend  all  the  way  across  the  section,  and  are  readily  recognizable. 
The  number  of  cleavage  cracks  appears  to  have  no  connection 
with  the  character  of  the  cleavage.  Brittle  minerals  show  a 
larger  number  in  general  than  those  that  are  less  brittle  even 
though  the  latter  have  a  more  perfect  cleavage.  In  rocks  which 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      191 


have  been  shattered  by  erogenic  processes,  cleavage  is  particularly 
noticeable.  Figs.  200  to  202  show  the  difference  in  general 
appearance  of  perfect,  distinct,  and  imperfect  cleavages. 

Fibrous  cleavage  is  characteristic  in  cyanite  and  gypsum,  and  may  be 
recognized  by  an  aggregation  of  extremely  fine  lines  in  one  place  on  the 
section.  Parting  faces  appear  different  from  the  ordinary  kind  of  cleavage 
because  they  are  regular  and  straight,  as  in  diallage. 

Besides  these  appearances,  based  on  the  inner  structure  of  the  rock- 
forming  minerals,  cracks,  which  are  more  or  less  straight  and  of  an  inciden- 
tal nature,  are  to  be  observed  in  the  thin  sections  of  many  minerals.  In  gar- 
net, for  example,  a  checked  or  cracked  appearance  is  extremely  widespread 
and  the  short  cracks  cut  through  the  whole  section  in  the  same  manner  with- 
out any  relation  whatever  to  the  orientation  of  the  garnet  individuals. 
Long  needles,  e.g.,  of  apatite  or  sillimanite,  tend  to  show  a  distinct  cross 
fracture  which  could  easily  be  confused  with  cleavage.  Such  crystals  are 
very  easily  divided  by  breaking  and  even  the  movement  of  a  liquid  stream 
of  lava  is  sufficient  to  break  a  prismatic  or  tabular  feldspar  crystal  and  the 
parts  may  be  found  in  a  thin  section  in  close  proximity  to  one  another 

It  is  more  difficult  to  explain  the  fact  that  some  minerals,  e.g.,  pyroxene 
in  unaltered  andesite,  do  not  show  complete  extinction  but  appear  to  be 
compressed  into  columns  that 
extinguish  differently,  i.e.,  they 
possess  undulatory  extinction.  This 
can  only  be  caused  by  fracturing 
which  has  resulted  during  the 
crystallization  of  the  magma  and 
is  called  protoclase.  Analogous 
disturbance  of  the  optical  behavior 
is  far  more  common  in  plutonic 
and  in  all  possible  metamorphic 
rocks.  Certain  brittle  constitu- 
ents often  show  a  wavy  or  undu- 
latory extinction  in  which  uniaxial 
crystals,  e.g.,  quartz,  frequently 
become  biaxial.  This  very  readily 
changes  into  fracture  and  the 
grains  of  quartz  or  of  olivine  ap- 
pear as  an  aggregate  of  columnar 

crystals,  which  show  the  extinction  directions  slightly  different,  but  sharply 
separated,  so  that  the  mineral  under  certain  conditions  appears  quite  fibrous. 

The  columns,  further,  may  be  cross  fractured  and  along  the 
contact  surfaces  of  the  grains  a  fine  friction  material  is  formed. 
Homogeneous  quartz  grains  are  changed  to  a  very  fine  aggregate 
of  particles  so  intimately  dovetailed  into  each  other  that  they 
still  form  quite  a  compact  sand.  This  is  seen  around  the  larger 
grains  and  is  called  mortar  structure,  Mortelstruktur,  Fig.  203. 


FIG.  203. — Mortar  Structure  in  Quartz. 


192  PETROGRAPHIC  METHODS 

It  is  not  certain  how  much  of  this  mechanical  deformation  is  due 
to  stresses  dominant  during  crystallization  and  how  much  is  due 
to  orogenic  pressure  acting  at  a  later  period.  However,  the  whole 
phenomenon  is  generally  termed  cataclase. 

This  deformation  often  produces  a  real  characteristic  appear- 
ance in  minerals  with  a  good  cleavage.  Individuals  of  horn- 
blende, for  example,  may  be  divided  up  into  cleavage  prisms  so 
that  they  behave  in  cross  section  like  a  packet  composed  of 
rhombs.  Minerals  with  gliding  planes  like  mica  split  parallel 


FIG.  204. — Bent  Mica  Plates,  Mica  Schist.          FIG.  205. — Marble  with  Bent  Lamellae. 

to  these  under  the  influence  of  orogenic  pressure,  and  calcite 
under  the  same  conditions  assumes  a  fibrous  character  due  to 
very  fine  twinning  lamination.  These  minerals  are,  however, 
far  more  flexible  than  the  ones  first  mentioned,  especially  mica 
and  calcite,  and  to  a  less  degree  feldspar  also.  They  can  be 
bent  and  crumpled  considerably  without  fracturing,  Figs.  204  and 
205,  but  finally  when  the  action  becomes  too  great  they  also  are 
deformed  in  the  same  manner  as  quartz,  i.e.,  assume  the  Mortel- 
structur. 

Intergrowth  and  Inclusions. — Regular  and  irregular  inter- 
growths  of  various  minerals  are  widespread.  It  is  necessary  to 
distinguish  between  poikilitic  intergrowth  and  peripheral  develop- 
ment. In  the  first  case  two  homogeneous  minerals  mutually 
penetrate  each  other.  Pegmatitic  intergrowth  of  quartz  and 
feldspar,  graphic  granite,  Fig.  206,  and  perthitic  intergrowth  of 
various  feldspars  (see  feldspar  group)  are  typical  examples. 
Peripheral  growth  is  most  frequent  in  related  minerals  as  ortho- 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      193 

rhombic   and   monoclinic   pyroxenes,    monoclinic    and   triclinic 
feldspars,   or  hornblende  and  pyroxene.     It  is  also  known  in 


FIG.  206. — Graphic  Granite  from  Jekaterinburg,  Ural. 

minerals  that  are  not  at  all  related  to  each  other,  in  gabbros 
where  ilmenite  is  frequently  surrounded  by  a  halo  of  biotite  and 
olivine  by  a  halo  of  hypersthene  or  garnet,  etc. 


FIG.  207. — Ocellary  Structure,  Leucitophyre  FIG.  208. — Quartz  Eye  with  a  Halo  of  Glass  Rich 
Olbriick,  Eifel.  in  Microlites.     Basalt,  Neuhaus,  Oberpfalz. 

The  borders  around  certain  minerals  often  show  characteristic  phenomena. 
For  example,  in  volcanic  glasses,  a  dark  colored  fringe  may  be  seen  around 
feldspar  crystals  and  a  lighter  colored  one  around  pyroxene.  This  is 
especially  noticeable  in  leucite  rocks  where  acicular  crystals  of  aegirine 
grow  around  the  larger  leucite  individuals,  ocellary  structure,  Fig.  207,  or 
13 


194  PETROGRAPHIC  METHODS 

in  eclogites  in  which  the  garnets  protrude  into  their  surroundings  like 
pseudopods.  Among  such  appearances,  which  are  classed  together  as 
concentric  structures,  the  quartz  lenses  of  basic  eruptive  rocks,  Fig.  208,  are 
especially  noteworthy  but  they  have  an  entirely  different  origin. 

Crystal  inclusions  are  distinguished  from  intergrowths,  when 
the  foreign  mineral  that  is  included  in  another  crystal  is  less  in 
quantity  or  possesses  its  own  crystal  form  against  the  including 
substance,  as  quartz  in  hornblende  in  Fig.  209.  The  inclusions 
may  be  variously  orientated  or  they  may  be  arranged  parallel 
to  some  crystallographic  direction  of  the  including  mineral. 

Single  grains  of  various  miner- 
als may  also  be  included  by 
another.  In  all  these  cases  the 
relations  between  the  inclusion 
and  the  including  mineral  are 
undoubtedly  not  so  intimate  as 
in  the  case  of  an  intergrowth. 
Gas,  liquid,  and  glass  inclu- 
sions are  also  observed  but 
their  characteristics  have  been 
briefly  described  in  part  I, 
page  48. 

The  ability  of  various  rock 

FIG.  209.-Perforated  Hornblende  Granulite,    Constituents    to    take    UP    inclll- 

Ampe,  Ceylon.  sions   is  quite   variable.     It  is 

quite    subordinate     in    quartz 

but  plagioclase  occurring  right  beside  it,  often  shows  abundant 
inclusions.  This  is*  also  particularly  noticeable  in  garnet,  cya- 
nite,  staurolite,  calcite,  etc.,  where  the  quantities  of  inclusions 
may  be  so  large  as  to  exceed  the  mass  of  the  including  mineral. 

The  significance  of  the  inclusions  as  to  the  conditions  of  the 
formation  of  rocks  was  early  recognized,  especially  in  the  case  of 
liquid  inclusions  in  the  quartz  of  granites,  which  point  to  an 
hydatopyrogenic  origin. 

With  a  low  magnification,  cloudy  bands  may  be  observed  penetrating  the 
quartz  grains  of  granite  without  any  reference  to  the  orientation  of  the 
grains.  With  stronger  magnification  they  are  resolved  into  swarms  of 
thousands  of  the  most  minute  liquid  inclusions  with  gas  bubbles  often  in 
extremely  violent  motion.  The  liquid  is  sometimes  water,  or  an  aqueous 
solution,  and  sometimes  liquid  carbon  dioxide.  The  latter  is  best  tested 
by  heating  the  preparation  above  the  critical  temperature  of  carbon  dioxide, 
i.e.,  above  31°  C.  At  this  temperature  the  gas  bubble  disappears  suddenly 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      195 

accompanied  by  a  violent  disturbance  of  the  liquid,  and  the  liquid  inclusion 
becomes  a  gas  inclusion.  It  can  also  be  proved  analytically  that  it  is 
really  carbon  dioxide. 

Liquid  inclusions  do  not  always  have  such  small  dimensions.  Large 
irregular  cavities  filled  with  liquid  are  sometimes  seen  in  olivine  crystals  in 
peridotites.  In  contact  rocks  the  liquid  inclusions  frequently  fill  negative 
crystals,  i.e.,  cavities  with  the  same  shape  as  the  crystal,  and  of  considerable 
size.  They  are  sometimes  apparent  macroscopically  in  quartz  crystals, 
and  particularly  in  halite. 

Liquid  inclusions  are  found  especially  in  minerals  in  plutonic 
rocks  and  in  contact  rocks,  and  these  minerals  do  not  often 
contain  glass  inclusions.  Glass  inclusions  are  characteristic  for 
extrusive  rocks,  and  it  may  also  be  observed  that  when  they  are 
included  in  light  minerals,  they  are  darker  and  in  dark  minerals 
lighter  than  the  glassy  basis  of  the  rock  itself.  The  form  of 
glass  inclusions  is  sometimes  irregular,  sometimes  oval,  or  in  the 
form  of  negative  crystals  with  rounded  edges.  Their  dimensions 
are  never  as  small  as  those  of  liquid  inclusions,  but  they  rarely 
exceed  microscopic  size. 

Inclusions  of  pure  glass  are  found  mostly  in  acid  and  inter- 
mediate extrusive  rocks  and  even  also  in  such  rocks  in  which  the 
glassy  basis  has  been  completely  devitrified  by  secondary  altera- 
tion. Sometimes,  however,  the  alteration 
has  effected  the  glass  of  the  inclusions  and 
their  character  can  only  be  recognized  by 
the  outer  form.  Corresponding  to  the 
greater  ease  with  which  basic  magmas 
crystallize,  real  glass  is  less  often  observed 
as  an  inclusion  in  basic  rocks,  the  glass 
tending  to  be  more  or  less  filled  with  micro- 

. . ,  ,  .    ,         ,         ,     .,  ~       ,  FIG.  210.— Leucite  with 

lites   which  cloud  it.      Such  products  are  siag  inclusions, 

better  termed    slag    inclusions.      In  lava, 
which   has   solidified   rapidly,    they  entirely  fill  the  plagioclase 
individuals  and  in  leucite  crystals  they  cause  a  characteristic 
appearance  by  their  radial  or  zonal  arrangement,  Fig.  210. 

The  arrangement  of  the  inclusions  is  often  very  characteristic. 
Aggregations  in  the  center  or  in  border  zones  are  especially 
prevalent,  as  is  also  zonal  arrangement.  The  plagioclases  a"re 
frequently  built  up  of  zones  alternately  rich  and  poor  in  inclu- 
sions. In  garnet  the  inclusions  are  sometimes  orientated 
parallel  to  crystallographic  directions  and  sometimes  tangential 
to  them.  In  certain  pyroxenes,  in  olivine,  and  in  the  labradorite 


196  .  PETROGRAPHIC  METHODS 

of  gabbros,  brown  flakes  and  rods,  which  are  perhaps  ilmenite, 
are  regularly  arranged  and  often  cause  a  metallic  chatoyancy  in 
the  mineral  visible  to  the  naked  eye.  The  arrangement  of 
graphitic  dust  in  certain  minerals  of  contact  rocks  is  especially 
typical,  particularly  in  andalusite.  The  variety  known  as  chias- 
tolite  presents  the  appearance  shown  in  Fig.  211.  In  other 
minerals,  which  are  likewise  often  rich  in  inclusions,  as  cyanite 
or  staurolite,  no  regularity  in  the  arrangement  of  the  inclusions 
is  to  be  noted. 

One  type  of  arrangement  of  inclusions,  which  is  classed  with 
the  characteristic  structures  of  contact  rocks,  is  especially  inter- 
esting. If  a  contact  schistose  rock  is  cut  across  the  schistocity 


FIG.     211. — Chiastolite  in  Chiastolite  FIG.  212. — Helicoidal   Structure. 

Schist  Gunildrud  on  Ekernsee,  Norway.  Cordierite  Gneiss,  Bodenmais. 

(After  E.  Cohen.) 

and  observed  in  thin  section  an  irregular  crystalline  aggregate 
appears.  However,  the  bands  of  inclusions  are  often  inter- 
woven in  the  principal  constituents  and  reveal  the  original 
schistocity  with  all  its  contortions  very  plainly.  This  is  called 
helicoidal  structure.  In  cordierite  hornfels,  Fig.  212,  these  bands 
consist  of  sillimanite,  biotite,  and  ilmenite,  in  other  cases  of 
graphite  dust,  and  in  still  others  of  small  quartz  grains,  but  in 
spite  of  the  varying  composition  the  general  appearance  remains 
the  same  and  they  are  therefore  extremely  characteristic. 

A  large  amount  of  inclusions  in  certain  constituents,  especially 
the  larger  ones,  is  characteristic  for  contact  rocks.  Such  is 
particularly  the  case  in  garnet,  in  which  sometimes  the  center  of 
the  crystal  is  a  compact  mass  of  inclusions  surrounded  by  a  ring 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      197 


of  garnet,  perimorph,  Fig.  213,  and  sometimes  the  irregularly 
arranged  inclusions  constitute  the  greater  part  of  the  crystal  and 
are  interwoven  with  a  network  of  fine  veins  of  the  including 


FIG.  213. — Garnet  Perimorph.     Eclogite, 
Kleinitz  in  Grosvenedig. 


FIG.  214. — Sieve  Structure.     Mica  Schist, 
Zopetspitze,  Grosvenedig. 


mineral,  sieve  structure,  Fig.  214.  The  external  form  of  such 
crystals  so  rich  in  inclusions  may  be  perfectly  normal. 

The  occurrence  of  pleochroic  halos  must  be  mentioned.  They  occur 
around  inclusions  of  certain  minerals  containing  zirconium,  tin,  titanium, 
and  some  of  the  rare  earths,  when  included  in  certain  other 
minerals,  see  Table  19,  No.  IV. 

They  may  be  best  observed  when  the  direction  of  strongest 
absorption  of  the  including  crystal  is  parallel  to  the  vibra- 
tion direction   of  the  polarizer  and  then  they  appear  as 
quite  small  zones  not  sharply  separated  from  the  surround- 
ing mineral,  Figs.  215  and  216.     The  pleochroic  halos  show 
a  much   stronger  ab- 
sorption of  light  than 
does   the    rest  of  the 
crystal,  yellow  to  deep 
brown     colors     being 
especially       common. 
If,  on  the  other  hand, 
the  direction  of  least 
absorption  is  parallel 
to    the    polarizer   the 
phenomenon      almost 

completely  disappears.  The  form  of  the  halo  varies  greatly  and  is  dependent 
upon  the  form  of  the  inclusion.  It  is  very  significant  that  other  minerals 
than  those  included  in  Table  19,  e.g.,  the  needles  of  apatite  in  biotite,  Fig. 
216,  do  not  show  such  halos.  Pleochroic  halos  may  be  of  a  secondary  origin, 
e.g.,  when  augite,  which  does  not  show  them,  alters  to  hornblende.  In 


II 

i 

I 

m 
m 

i': 

1 

& 

If 

ii 

FIG.  215.  FIG.  216. 

Pleochroic  Halos  around  Inclusions  of  Zircon  in 
Tourmaline.  Biotite. 


198  PETROGRAPHIC  METHODS 

general  the  optical  properties  of  a  crystal  are  quite  strongly  modified  in 
the  pleochroic  halos.  The  indices  of  refraction  are  raised,  the  double  refrac- 
tion may  become  much  lower  or  may  be  raised  considerably,  and  in  biaxial 
crystals  a  stronger  dispersion  of  the  optic  axes  is  generally  to  be  observed 
in  the  halos.  All  these  phenomena  do  not  indicate  that  the  coloring  is 
due  simply  to  an  organic  pigment  but  much  more  that  it  is  due  to  an  iso- 
morphous  mixture  in  which  the  elements,  that  are  predominant  in  the  in- 
clusions, play  a  considerable  role.  The  ease  with  which  these  halos  can  be 
destroyed  by  heating  does  not  in  any  way  oppose  this  view  for  that  is  a 
common  property  of  silicates  colored  dark  by  a  content  of  titanium,  e.g., 
melanite,  and  also  of  numerous  occurrences  of  dark  rutile,  cassiterite, 
zircon,  etc. 

Pseudomorphs. — Alteration  of  rock-forming  minerals  is  exceedingly  com- 
mon, but  very  frequently  the  form  and  also  a  considerable  portion  of  the 
original  mineral  remains  unchanged  indicating  clearly  the  origin  of  the 
pseudomorphs.  Most  of  the  pseudomorphs  are  not  the  result  of  the  action 
of  ground  waters.  Pseudomorphs  due  to  weathering  are  comparatively 
rare.  The  most  of  these  occurrences  are  to  be  ascribed  to  the  post- volcanic 
action  of  magmatic  waters.  They  are,  therefore,  most  important  in  eruptive 
rocks  and  in  regions  where  they  have  effected  contact  metamorphism. 
These  regions  are  the  characteristic  habitats  of  pseudomorphs. 

Pseudomorphs,  which  exhibit  a  new  composition  in  an  old  form,  some- 
times consist  of  a  newly  developed,  homogeneous  crystal,  e.g.,  uralite.  In 
most  cases,  however,  they  are  composed  of  a  dense  aggregate  which  may 
still  show  the  cleavage  and  structure  of  the  original  mineral,  thus  bastite, 
or  may  form  an  irregular  cluster,  which  presents  the  phenomenon  of  aggre- 
gate polarization  in  polarized  light,  e.g.,  liebenerite. 

An  important  group  of  alterations  belongs  in  this  class,  in  which  simple 
molecular  rearrangement  takes  place  without  change  of  chemical  com- 
position— paramorphs.  One  of  the  most  important  chemical  geological 
processes  belongs  here.  It  is  the  very  widespread  transformation  of 
aragonite  into  calcite  in  the  fossilization  of  organic  lime  skeletons.  Some- 
times the  original  structure  is  retained,  while  at  other  times  it  is  com- 
pletely obliterated  during  the  process.  The  transformation  of  minerals 
free  from  water  into  those  containing  water  is  especially  frequent.  There 
are  two  processes  of  this  kind  which  take  place  very  extensively,  e.g.,  the 
formation  of  gypsum  and  of  serpentine.  An  increase  of  volume  takes 
place  which  produces  great  geological  disturbances  and  its  effects  are  even 
noticeable  on  a  small  scale  in  the  fracturing  of  the  surrounding  minerals, 
which  are  traversed  by  radial  cracks,  Fig.  217. 

Alteration  begins  for  the  most  part  on  the  border  and  progresses  into  the 
interior  along  the  cleavages  and  gliding  planes,  expanding  these  more  and 
more  until  a  peculiar  network  results,  Fig.  218,  within  which  a  residue  of 
the  original  mineral,  still  unaltered  but  greatly  clouded,  is  often  preserved. 
In  the  normal  case  the  newly  developed  substance  is  confined  strictly  to  the 
border  of  the  original  crystal,  but  the  texture  of  the  altered  rock  may  be 
recognized  excellently,  palimsest  structure.  The  former  shape  is  lost  in  the 
newly  developed  minerals  only  when  the  alteration  is  especially  extensive 
and  general.  Secondary  formations  develop  abundantly  beyond  the 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      199 

former  borders  of  the  minerals  and  the  texture  of  the  rock  is  more  and  more 
confused.  In  exceptional  cases  the  alteration  begins  in  the  middle  of  an 
individual,  but  only  when  the  crystals  are  zonal  and  the  nucleus  is  more 
susceptible  to  alteration,  as  in  the  plagioclases  of  eruptive  rocks. 

One  must  guard  himself  against  confusing  alteration  products  with  in- 
clusions. Corresponding  to  the  manner  of  formation,  alteration  products 
have  a  form  dependent  upon  cleavage  cracks  and  other  discontinuities  in  the 
crystal.  They  form  a  more  or  less  regular  network  in  a  crystal  which  has 


FIG.    217. — Radial   Cracks   in   Plagioclase    FIG.  218. — Mesh  Structure  in  Serpentinized 
produced  by  Serpentinization  of  Olivine.       Olivine.     Trogen  near  Hof ,  Fichtelgebirge. 

suffered  alteration,  but  single  constituents  of  the  secondary  substance  lack 
characteristic  form.  Inclusions,  however,  can  have  definite  forms  and  may 
be  in  definite  orientation  bearing  no  relation  to  the  cleavage  cracks.  They 
do  not  form  continuous  veins  but  are  in  small  sharply  defined  crystals, 
which  are  completely  imbedded  in  an  unaltered  clear  transparent  mineral. 

Directions  for  the  Use  of  the  Descriptive  Section. — The  optical 
properties  of  rock-forming  minerals  are  taken  as  a  basis  for  the 
presentation  in  the  descriptive  portion  and  in  the  accompanying 
tables.  A  thorough  knowledge  of  the  methods  of  determining 
these  properties,  as  is  set  forth  in  Part  I,  must  be  presupposed 
and  the  following  paragraphs  will  only  recapitulate  the  points 
particularly  important  in  the  use  of  this  portion. 

It  may  be  remarked  that,  in  so  far  as  the  transparent  minerals 
are  concerned,  the  presentation  of  the  properties  in  the  descriptive 
section  will  only  supplement  the  statements  in  the  tables  so  that 
both  must  always  be  used  jointly.  The  arrangement  in  the  text 
is  the  same  as  in  the  tables  and  the  number  after  the  name  of 
each  mineral  in  the  descriptive  section  indicates  the  page  of 
the  tables  where  the  mineral  is  to  be  found  and,  vice  versa,  the 


200  PETROGRAPHIC  METHODS 

number  in  the  first  column  of  the  tables  refers  to  the  page  in  the 
text  giving  the  description  of  the  mineral. 

The  systematic  classification  of  rock-forming  minerals  must  be 
different  from  that  used  in  mineralogy  because  of  the  methods 
used  to  determine  them.  A  chemical  system,  which  miner- 
alogists find  so  useful,  has  no  significance  in  this  case.  The 
classification,  which  is  adhered  to  in  this  text,  has  proved  to  be 
especially  valuable  and  divides  minerals  into  four  groups: 

1.  Opaque  minerals. 

2.  Isotropic  minerals. 

3.  Uniaxial  minerals. 

4.  Biaxial  minerals. 

This  classification  is  based  upon  those  properties  which  can  generally  be 
determined  quickly  and  positively  by  simple  methods.  In  certain  cases 
some  difficulties  are  encountered,  especially  by  a  beginner,  because  of  the 
overlapping  of  the  groups,  but  an  experienced  observer  will  soon  overcome 
this.  For  example,  hematite  and  ilmenite  are  found  in  the  column  of  opaque 
minerals,  but  in  certain  instances  they  occur  in  fine  transparent  double 
refracting  scales.  Chromite  is  also  quite  frequently  transparent  with  a 
brown  color.  On  the  other  hand,  some  minerals  from  the  other  three 
groups  are  often  so  deeply  colored  that  they  do  not  transmit  light  except 
in  the  thinnest  sections,  as  certain  varieties  of  rutile  or  lievrite,  and  cossyrite. 
They  may  be  distinguished  from  opaque  minerals,  however,  by  an  entire 
lack  of  metallic  luster  in  reflected  light.  This  is  especially  so  with  the  last 
two  minerals. 

Certain  of  the,  isotropic  minerals  are  not  isotropic  in  all  cases.  Such  are 
perovskite,  garnet,  leucite,  and  analcite.  However,  these  generally  show 
a  well  characterized  regular  structure  of  several  double  refracting  parts  so 
that  one,  with  experience  at  least,  will  scarcely  ever  be  in  doubt  as  to  how 
to  classify  it.  Some  double  refracting  minerals  may  also  be  present, 
whose  birefringence  is  so  small  that  they  could  be  overlooked  in  a  cursory 
examination  of  a  section,  e.g.,  apatite,  eudialyte,  zoisite,  etc.  Optical 
anomalies  are  very  common  in  uniaxial  substances  and  they  generally 
appear  biaxial  with  a  small  optic  angle  in  convergent  light.  There  are 
also  biaxial  minerals  in  which  the  optic  angle  is  nearly  zero  and  they  give 
the  impression  of  being  uniaxial,  thus,  mica.  Any  scheme  of  classification 
based  upon  a  series  of  external  features  will  show  such  discrepancies,  but, 
as  already  remarked,  they  can  be  easily  overcome  and  with  a  little  practice 
any  one  can  readily  apply  the  division  into  these  four  groups. 

That  property  which  is  first  apparent,  viz.,  refraction,  is  used 
as  a  basis  for  further  subdivision  of  the  transparent  minerals,  so 
that  in  each  of  the  three  groups  the  minerals  are  generally  arranged 
according  to  the  decreasing  index  of  refraction.  However, 
this  principle  is  carried  out  only  on  general  lines  because  there 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      201 

are  no  means  of  measuring  the  index  of  refraction  of  a  mineral 
embedded  in  Canada  balsam  accurately.  The  necessity  of 
treating  various  isomorphous  groups  as  units  gives  rise  to 
numerous  deviations  because  there  are  sometimes  great  differ- 
ences in  the  indices  of  refraction  of  various  members  of  such  a 
group,  but  this  will  scarcely  ever  give  rise  to  confusion. 

The  observation  of  the  relief  of  a  mineral  in  a  thin  section  gives 
a  clue  to  the  position  in  which  that  mineral  is  to  be  found  in  the 
table.  Table  No.  3  can  be  used  for  an  approximate  classification 
of  the  mineral. 

The  index  of  refraction  is  generally  given  to  the  third  decimal  place, 
which  is  unreliable  in  many  cases.  The  determinations  of  various  in- 
vestigators of  ten  show  variations  of  several  units  in  the  third  decimal  place, 
even  in  minerals  which  have  a  comparatively  simple  composition.  In 
minerals  belonging  to  an  isomorphous  series  of  many  members,  the  variations 
in  the  determinations  are  much  greater,  e.g.,  almandine  from  Silberbach 
in  the  Fichtelgebirge,  n  =  1.761;  from  Falls  in  the  Fichtelgebirge,  n  =  1.770; 
from  India,  n  =  1.791  — 1.800.  Three  values  are  given  for  blue  spinel 
from  Ceylon,  n  =  1.719;  n  =  1.720;  and  n  =  1.726.  It  seems  useless,  there- 
fore, to  give  values  to  the  fourth  decimal  place. 

Table  18  gives  a  tabulation  of  the  minerals  treated  in  this 
text  according  to  their  double  refraction  and  the  interference 
colors  dependent  upon  it,  as  arranged  by  Michel-LeVy  and 
Lacroix.  The  values  given  there  represent  the  difference  be- 
tween the  maximum  and  the  minimum  indices  of  refraction,  i.e., 
f—  a,  and  generally  are  the  mean  where  several  values  are  to  be 
found.  The  apparent  extreme  values  are  given  only  where  the 
variations  in  the  double  refraction  are  especially  large,  as  in 
chloritoid  and  titanite.  The  table  may  be  used  in  the  following 
manner.  The  thickness  of  the  section  is  measured  (see  Part  I, 
page  45)  and  then  a  cross  section  is  sought  as  nearly  parallel  to  the 
optic  axis  or  to  the  optical  plane  as  possible.  This  can  be  tested 
in  convergent  polarized  light.  The  interference  color  of  the 
section  gives  a  very  accurate  means  of  determining  the  mineral. 
If,  for  example,  by  means  of  the  low  yellowish-white  interference 
color  of  a  section  of  quartz  properly  orientated,  the  thickness  of 
the  slide  is  found  to  be  0.03  mm.;  while  another  mineral,  which  is 
also  colorless,  shows  yellow  of  the  second  order  for  its  highest 
interference  color,  the  vertical  line  corresponding  to  this  color 
is  followed  to  its  point  of  intersection  with  the  horizontal  line 
corresponding  to  the  thickness  0.03  mm.  The  diagonal  through 
this  point  gives  the  double  refraction  of  the  mineral  in  question, 


202  PETROGRAPHIC  METHODS 

which  in  this  particular  case  is  approximately  0.030.  The  dis- 
tinction between  the  few  minerals  to  which  the  investigation  has 
thus  been  limited,  can  be  made  from  a  more  careful  study  of  the 
other  optical  properties. 

The  color  of  a  mineral  is  highly  characteristic  and  useful  in  its 
determination.  Table  18  is  arranged  according  to  this  prin- 
ciple. Many  minerals,  which  are  macroscopically  colored,  are 
entirely  colorless  in  thin  section  and  in  all  cases  the  thinner  the 
section  the  lighter  the  color  and  more  difficult  is  it  to  determine. 
The  cause  of  the  color  varies  greatly.  Sometimes  a  character- 
istic relation  to  the  chemical  composition  can  be  recognized,  as  in 
the  pyroxenes  and  amphiboles.  Sometimes  the  color  changes 
without  any  apparent  variation  in  the  chemical  composition.  In 
this  case,  the  color  is  produced  by  an  admixture  of  an  extremely 
small  amount  of  highly  colored  material  as  seen  in  garnet.  In 
a  third  case,  microscopic  investigation  reveals  the  fact  that  in- 
clusions are  the  cause  of  color,  e.g.,  haiiyne  and  bronzite.  The 
color  in  this  case  is  quite  variable  and  many  such  minerals  are 
only  colored  occasionally.  The  color  is  not  always  uniform  in 
one  section  of  a  crystal.  Zonal  alternations  or  a  very  irregular 
speckled  distribution  of  the  color  are  very  common. 

The  presence  or  absence  of  pleochroism  is  likewise  a  highly 
characteristic  feature.  The  comparative  depths  of  color,  i.e., 
absorption,  which  appears  along  certain  axes,  is  quite  constant 
for  a  mineral  group,  and  for  this  reason  the  absorption  symbols 
are  usually  given  in  the  tables.  The  colors  of  different  axes, 
on  the  other  hand,  vary  within  wide  limits  and  are  only  given 
where  they  are  more  or  less  constant. 

A  number  of  minerals  are  included  in  this  part  which  occur 
only  rarely  as  rock  constituents.  They  are  distinguished  from 
the  more  important  by  smaller  type.  The  most  of  these  are  not 
encountered  by  a  beginner  and  are  only  observed  in  the  course 
of  advanced  study. 

Only  those  relations  are  considered  which  are  of  value  to  petro- 
graphers.  All  purely  chemico-mineralogical  and  otherwise 
theoretical  considerations  are  omitted.  Likewise,  the  crystal- 
lographic  development,  which  is  of  subordinate  significance  in  the 
determination  of  rock-forming  minerals,  is  only  discussed  in  a 
cursory  manner.  Complicated  crystal  drawings  are  avoided  and 
in  their  place  are  numerous  optical  sketches  for  various  minerals. 

In  explanation  of  the  figures  it  may  be  said  that  a,  b,  c  are  the 


DEVELOPMENT  OF  ROCK  CONSTITUENTS      203 

crystallographic  axes;  o,  B,  c  are  the  principal  vibration  direc- 
tions; a,  /?,  f  are  the  corresponding  indices  of  refraction;  A  and  B 
are  the  optic  axes. 

A  circle  with  a  cross  indicates  the  point  of  emergence  of  the 
optic  axis  of  a  uniaxial  mineral.  The  point  where  an  optic  axis 
emerges  from  a  biaxial  mineral  is  indicated  by  a  curve,  printed 
heavily  where  the  dispersion  is  great.  These  indicate  by  their 
position  and  distance  apart  the  location  of  the  axes  in  the  section 
in  question.  The  apparent  optic  angle  is  often  indicated  on  an 
important  crystal  face  or  cleavage  face,  if  it  is  parallel  to  the 
optic  plane  and  this  may  be  a  characteristic  property  of  the 
mineral. 

The  possibility  of  confusing  various  minerals  is  only  con- 
sidered in  their  descriptions  when  the  slight  differences  in  the 
optical  properties  might  be  easily  overlooked.  For  example,  the 
minerals  perovskite  and  melanite  are  sometimes  difficult  to  distin- 
guish in  spite  of  the  great  difference  in  the  indices  of  refraction, 
because  they  are  very  much  higher  than  that  of  Canada  balsam. 
Nepheline  and  apatite,  on  the  other  hand,  are  not  considered  as 
similar  minerals,  although  their  indices  of  refraction  are  nearly 
the  same,  but  the  difference  can  readily  be  observed  because  they 
both  lie  in  the  neighborhood  of  that  of  the  Canada  balsam. 


CHAPTER  XI 
Descriptive  Section 

As  already  mentioned,  the  best  classification  of  rock-forming 
minerals  embraces  the  following  four  groups:  1.  Opaque  Minerals. 
2.  Isotropic  Minerals.  3.  Uniaxial  Minerals.  4.  Biaxial  Minerals. 
Those  properties  which  are  first  apparent  in  microscopical  inves- 
tigation are  made  the  basis  for  this  classification.  A  classifica- 
tion according  to  crystal  systems  seems  to  be  less  commendable 
because  it  is  not  always  possible  to  determine  the  system  in  a 
thin  section,  especially  in  the  case  of  hexagonal  and  tetragonal 
minerals.  Such  a  classification  would  also  necessitate  a  division 
of  the  most  important  isodimorphous  groups,  which  can  be 
treated  as  a  whole  in  the  classification  proposed. 

i.  Opaque  Minerals 

Included  with  the  opaque  minerals  are  those  which  are  ordinarily  not 
transparent  in  thin  section.  They  have  more  or  less  of  a  metallic  luster 
macroscopically  and  in  reflected  light  under  the  microscope.  Since  from 
their  nature  further  optical  determinations  are  impossible,  microscopic 
distinctions  cannot  be  made  safely  if  they  have  the  same  surface  color,  and 
are  without  crystal  form. 

Some  opaque  minerals  are  cubic  and  some  hexagonal.  If  they  possess 
crystal  forms,  those  belonging  to  the  first  system  show  three-,  four-  or  six- 
sided  cross  sections  which  are  isometric,  while  the  typical  sections  of  those 
of  the  other  systems  are  hexagonal  or  lath-shaped,  corresponding  to  a  thin 
tabular  development. 

Pyrite 

Pyrite  (FeS2)  occurs  in  perfectly  bounded  crystals  of  various 
dimensions  consisting  usually  of  a  cube  a  striated  by  a  pyrito- 
hedron  e,  or  in  combination  with  it,  Fig.  219,  page  205.  It  also 
occurs  in  large  segregations  and  concretions,  sometimes  as  a 
cementing  material,  and  in  irregular  grains  and  fine  veins  as  an 
impregnation  in  rocks. 

It  dissolves  in  nitric  acid  but  hydrochloric  acid  scarcely 
attacks  it.  Hardness,  6.5;  sp.  gr.?  5.  Its  brass  yellow  color 

204 


DESCRIPTIVE  SECTION  205 

is  always  apparent  in  a  thin  section.     The  ease  with  which  it 
weathers   to   red   or   brownish-red   ferric   hydroxide    varies    in 
different   localities    and    causes   the    rock   to    decompose    very 
rapidly,  for  example  Alaunschiefer.     Pyrite  occurs  everywhere. 
When  it  is  present  as  a  constituent  of  eruptive  rocks  it  is  usually 
of  secondary  origin  and  its  occurrence  is  accompanied  by  many 
other    decomposition    phenomena,    such    as 
kaolinization,  propyllitization,  and  the  forma- 
tion of  greenstone.      It   is  frequently  found 
also  in  all- kinds  of  contact  rocks,  sometimes 
in  large  crystals  which  are  greatly  deformed 
if   the   rock   was   under  great  pressure,   and 
sometimes  in  massive  deposits  as  a  constit- 
uent    Of     the      Contact     ZOne.        It    OCCUrS    in     Cube  andPyritohedron. 

clastic  rocks,  principally  in  the  form  of  con- 
cretions in  clay  slates,  and  in  coal.     It  is  often  confused'  with  the 
more  bronze  colored  pyrrhotite  and  the  more  yellowish-green 
chalcopyrite,  but  it  can  be  distinguished  from  both  of  these  by 
its  superior  hardness. 

Pyrrhotite 

Pyrrhotite  (FeS),  as  a  rock  constituent,  is  found  principally  in  small 
grains  or  in  larger  dense  masses  and,  in  unaltered  gabbros,  occasionally  in 
tabular  crystals.  It  is  easily  dissolved  by  acids.  H.  =4;  sp.  gr.  =4.6 
Usually  strongly  magnetic.  Its  color  is  bronze  brown,  but  it  is  generally 
tarnished  or  entirely  altered  to  rust.  It  is  found  as  a  primary  constituent 
often  containing  nickel  in  basic  eruptive  rocks,  and  in  green  sclrsts  and 
amphibolites  derived  from  them;  also  in  contact  limestones.  It  often 
forms  comparatively  large,  dense  irregular  masses. 

Chalcopyrite 

Chalcopyrite  (CuFeS2)  is  quite  rare  as  a  true  rock  constituent.  It  is 
found  associated  with  other  sulphides  as  a  constituent  of  basic  eruptive 
rocks  and  green  schists,  derived  from  them,  and  in  granular  limestones. 
It  never  shows  crystal  form.  It  is  usually  in  very  irregular  grains  in 
which  well  developed  crystals  of  pyrite  have  grown.  The  distinction  by 
means  of  the  greenish-yellow  color  of  chalcopyrite  is  then  very  easy.  H.  =4; 
sp.  gr.  =4.2.  It  gives  a  copper  reaction. 

Galena 

Galena  (PbS),  associated  with  sphalerite,  is  not  a  rare  constituent  of 
contact-metamorphic  limestones.  Occasionally  it  occurs  in  well  developed 


206  PETROGRAPHIC  METHODS 


cubes,  Fig.  220,  but  more  often  it  is  in  grains  which  have 
a  light  lead  gray  color  and  an  extremely  brilliant  metallic 
luster.  It  generally  shows  very  perfect  cubical  cleavage 
in  reflected  light.  It  is  soft  and  often  has  a  smeared 
border,  like  graphite.  This  with  the  color  gives  it  the 

FIG.  220.— Cube,  appearance  of  molybdenite  but  it  differs  from  the  tabular 
individuals  of  molybdenite  in  being  more  compact. 

H.  =2.5;  sp.  gr.  =  7.5.     Soluble  in  nitric  acid  with  separation  of  sulphur. 

Metallic  Iron 

Metallic  iron  is  rare  in  terrestrial  rocks  and  occurs  only  in  angular  grains. 
It  is  always  alloyed  with  nickel  which  may  predominate,  as  in  awaruite. 
It  is  easily  soluble  in  acids  and  decomposes  Thoulet's  and  Klein's  solutions. 
In  the  latter  case  intense  blue  tungstic  acid  is  formed.  It  precipitates 
metallic  copper  from  copper  solutions.  H.=4;  sp.  gr.  =  7.8.  Strongly 
magnetic,  but  shows  no  polarity.  It  rusts  easily  in  moist  air.  While  it 
has  only  been  observed  on  the  earth  as  a  rare  constituent  of  the  most  basic 
eruptive  rocks,  it  is  known  to  occur  extensively  in  meteorites. 

Magnetite 

Magnetite  (Fe3O4),  which  often  contains  titanium — titanium 
magnetite — is  a  universal  constituent  of  rocks.  It  occurs  some- 
times in  octahedral  crystals,  Fig.  22 1,  or  twins,  Fig.  222,  some- 
times in  delicate  crystal  skeletons,  and  sometimes  in  irregular 
grains  or  compact  masses.  It  dissolves  easily  in  hydrochloric 
acid  especially  upon  addition  of  a  little  potassium  iodide  (dis- 
tinction from  hematite,  titanite,  graphite,  etc.).  H.  =  6;  sp. 


FIG.  221.— Octahedron.  FIG.  222. — Twin.     Spinel  Law. 

gr.  =5.2.  Strong  magnetism,  but  not  polar.  Quite  resistive 
to  weathering,  and  is;  therefore,  only  rarely  surrounded  by  a 
border  of  rust,  and  is  found  commonly  in  secondary  deposits — 
magnetite  sand.  It  appears  black  with  a  variable  strong 
metallic  luster.  The  simplest  means  of  isolating  it  is  with  a 
magnet.  It  may  be  distinguished  in  this  way  from  other 
similar  black  opaque  ores  and  from  graphite.  It  is  widely  dis- 


DESCRIPTIVE  SECTION  207 

seminated  in  the  eruptive  rocks,  particularly  in  the  basic  ones. 
It  is  usually  one  of  the  first  minerals  to  crystallize  and  so  is 
frequently  included  in  the  other  constituents,  especially  in  the 
colored  minerals.  Black  ores  are  very  frequent  in  the  form  of 
fine  dust  in  the  ground  mass  of  such  rocks.  Although  this  is 
probably  mostly  magnetite,  it  has  been  called  opazite  because  of 
the  difficulty  of  determining  it  positively.  It  is  quite  as  frequent 
in  all  other  groups  of  rocks. 

In  gabbros  black  compact  ores,  which  are  to  be  considered  partly  as 
titanium  magnetite  and  partly  as  ilmenite,  are  often  surrounded  with  a 
homogeneous  fringe  of  biotite.  Aggregates  of  magnetite  grains  are  widely 
disseminated  in  extrusive  rocks,  usually  mixed  with  augite,  which  some- 
times shows  the  form  and  cleavage  of  hornblende  or  biotite  quite  plainly, 
and  is  probably  produced  by  magmatic  resorption  of  them.  Intergrowth 
with  ilmenite  is  not  at  all  rare.  In  such  cases  the  magnetite  can  be  dissolved 
with  hydrochloric  acid. 

Chromite 

Chromite  [Fe(Al,Fe,Cr)2OJ  is  found  exclusively  in  olivine  rocks  in  sharp 
octahedral  crystals,  Fig.  221,  page  206,  or  in  compact  masses  often  of  con- 
siderable size.  It  is  one  of  the  oldest  products  of  crystallization  and  is 
very  frequent  in  sharp  crystals  as  inclusions  in  olivine  with  which  it  is 
almost  universally  associated.  Acids  attack  it  with  great  difficulty. 
H.=5.5;  sp.  gr.  =4.5.  Macroscopically,  it  is  brownish-black  with  only  a 
weak  metallic  luster,  which  appears  more  greasy  under  the  microscope. 
In  thin  section  it  is  frequently  transparent  with  a  brown  color  particularly 
on  the  edges.  Its  index  of  refraction  is  about  2.1.  It  is  unaffected  in 
the  alteration  of  olivine  to  serpentine  because  it  is  so  resistive.  A  bril- 
liant green  pleochroic  halo  of  chromeocher  is  often  to  be  observed  around 
the  crystals  of  chromite  in  serpentine.  It  is  easy  to  isolate  because  of  its 
high  specific  gravity  and  its  resistance  to  reagents,  and  it  can  be  tested  by 
the  bead  reactions. 

Hematite 

Hematite  (Fe2O3)  sometimes  occurs  in  compact  masses  or  in 
crystals  with  an  uncommonly  brilliant  metallic  luster,  Fig.  223. 
They  are  always  opaque  in  thin  section,  and 
appear  steel  gray  to  iron  black  in  reflected 
light.  Sometimes  it  is  in  thin  hexagonal 
plates  which  are  transparent  and  red.  In 
the  form  of  fine  dust,  hematite  is  the  most 

7  .  Hematite  Crystal. 

frequent  red  pigment  of  rocks.    It  is  difficultly 

attacked  by  hydrochloric  acid,  but  the  more  finely  divided  it 

is,  the  more  readily  is  it  dissolved.     It  is  not  magnetic  when  not 


208  PETROGRAPHIC  METHODS 

intergrown  with  magnetite.  H.  =6.5;  sp.  gr.  =5.25.  Streak 
red  to  reddish-brown.  It  is  very  resistive  to  weathering  but  is 
now  and  then  transformed  to  rust. 

Compact  nontransparent  hematite  is  found  in  all  kinds  of 
rocks.  In  eruptive  rocks,  it  is  more  frequent  in  the  acid  varieties 
into  which  it  is  often  introduced  by  fumaroles.  Thin  yellowish- 
red  plates  with  properties  like  those  of  mica;  which  without  fur- 
ther investigation  have  been  identified  as  hematite,  are  found  as 
orientated  inclusions  in  feldspar.  They  give  rise  to  the  red 
metallic  chatoyancy,  as  in  sunstone  (Sonnensteiri) .  It  is  not 
positively  determined  whether  these  are  hematite,  but  the  well 
developed  platy  crystals  with  brilliant  metallic  luster  in  reflected 
light,  which  are  so  widespread  in  contact  rocks,  positively  do 
belong  to  hematite.  The  thinner  individuals  are  transparent 
with  deep  red  color  and  have  strong  negative  double  refraction 
and  weak  pleochroism,  e  yellowish;  co  brownish-red.  Hematite 
is  known  as  a  red  pigment  in  all  rocks  from  granite  to  clay  slate 
and  is  especially  frequent  in  a  group  of  acid  porphyries  and  por- 
phyrites  that  have  been  devitrified  and  appear  felsitic.  No 
characteristic  properties  can  be  recognized  in  it  even  with  the 
strongest  magnification  and  so  this  pigment  sometimes,  with  a 
yellowish  or  brownish  color,  is  better  known  as  ferrite. 

Ilmenite 

Ilmenite  [(Fe,Ti)2O3]  is  more  frequently  found  in  crystals  than 
compact.  Two  varieties  can  also  be  distinguished  in  this 
mineral.  One  has  a  weak  metallic  luster,  is  iron  black,  and 
occurs  in  tabular  crystals  or  in  dense  compact  masses;  the  other 
is  micaceous,  brown  transparent,  and  has  a  submetallic  luster. 
Both  types  have  a  great  tendency  to  grow  in  skeletal  forms. 
Perhaps  the  ocherous  brown  pigment,  e.g.,  in  the  plagioclases  of 
certain  gabbros,  may  be  referred  to  ilmenite. 

It  behaves  like  hematite  toward  acids  and  the  magnet.  H.  = 
5.5;  sp.  gr.  =4.8  to  5.2.  Unlike  hematite  it  is  frequently  decom- 
posed and  is  then  real  characteristic.  It  is  coated  with  a 
clouded  film  of  leucoxene,  white  in  reflected  light,  consisting  of  a 
mixture  of  various  titanium  minerals  and  at  the  same  time  the 
cleavage,  which  is  not  very  apparent  in  the  fresh  mineral,  becomes 
plainly  visible.  Ilmenite  may  finally  be  entirely  replaced  by 
leucoxene.  Alteration  into  homogeneous  individuals  of  titanite 


DESCRIPTIVE  SECTION 


209 


or  rutile,  or  zonal  growths  with  these  minerals  are  not  rare. 
It  is  found  principally  in  eruptive  rocks  and  of  these  it  favors  the 
basic  and  those  rich  in  sodium.  The  micaceous  type  with  not 
very  strong  double  refraction,  optically  negative,  £  brown,  cu 
yellow,  is  confined  to  the  porphyric  eruptive  rocks.  Perhaps  the 
orientated  brown  tabular  inclusions,  which  produce  the  metallic 
chatoyancy  of  diallage,  hypersthene,  etc.,  belong  to  this  mineral. 


Graphite 

Graphite  (C)  is  found  in  granular  limestones  rarely  crystallized 
in  hexagonal  plates  with  a  brilliant  metallic  luster  and  steel  gray 
color.  It  generally  forms  shredded  or  flaky  aggregates,  which 
have  a  great  tendency  to  parallel  intergrowths  with  altered  mica, 
Fig.  224.  If  it  is  treated  with  nitric  acid  and  heated  on  a  plati- 
num foil,  it  swells  up  into  voluminous  worm-like  particles.  In 
other  rocks  the  individuals  are  more  compact  and  of  even  size 
and  have  an  egg-shaped  cross 
section  in  the  slide.  Such 
varieties  are  denser  and  do 
not  show  the  flaky  character 
of  graphite  to  the  naked  eye. 
They  do  not  swell  when  heated 
with  nitric  acid.  For  this  rea- 
son it  has  been  considered  as 
an  individual  mineral  called 
graphitite.  Graphite  is  a 
widely  disseminated  black 
pigment  of  crystalline  schists. 
As  such  it  is  as  fine  as  dust 
and  a  crystallographic  devel-  ,-,  00/l •  n  ... 

FIG.  224. — Graphite  m  Graphite  Gneiss  from 
Opment     Cannot     be     Observed  Pfaffenreuth  near  Passau. 

even  with  the  strongest  mag- 
nification. This  is  designated  as  graphitoid,  and  is  considered  as 
a  transition  product  to  amorphous  coal.  The  properties  of 
these  three  types  of  graphite  are  too  similar  to  permit  a  separa- 
tion to  be  made,  especially  since  they  are  related  by  all  possible 
transition  stages. 

Graphite  is  completely  insoluble  in  acids  and  molten  alkalies* 
If  it  is  melted  with  metallic  potassium,  and  ferrous  and  ferric 
salts  are  added  with  some  hydrochloric  acid,  Berlin  blue  is 

14 


210  PETROGRAPHIC  METHODS 

frequently  obtained.  This  is  a  proof  of  a  small  content  of  nitro- 
gen in  the  graphite.  In  a  mixture  of  fuming  nitric  acid  and 
potassium  chlorate,  it  is  oxidized  to  graphitic  acid,  which 
crystallizes  in  golden  yellow,  metallic,  hexagonal  plates.  They 
are  optically  negative  and  have  a  strong  double  refraction.  The 
coarser  flaky  varieties  of  graphite  burn  very  difficultly  but  when 
finely  divided  it  burns  quite  easily.  It  is  a  good  conductor  of 
heat  and  electricity.  Rocks  rich  in  graphite  feel  cold.  When 
held  in  a  zinc  holder,  it  precipitates  metallic  copper  from  a 
solution  of  copper.  H.  =  l;  sp.  gr.  =  2.3.  The  borders  of  its 
cross  sections  never  appear  sharp  on  account  of  its  softness. 
It  is  smeared  over  the  entire  section  in  fine  powder  during  the 
process  of  grinding  so  that  its  distribution  is  often  recognized 
with  difficulty.  This  property  is  one  of  the  best  characteristics 
of  graphite  compared  with  hematite  and  ilmenite,  which  are 
similar  minerals.  It  is  also  distinguished  from  them  by  its 
insolubility  and  its  ability  to  burn.  The  brilliant  metallic  luster 
of  flaky  aggregates  produced  by  a  very  perfect  cleavage  parallel 
to  the  base,  becomes  more  and  more  obscured  in  the  more  com- 
pact varieties.  However,  the  grayish-black  streak  always  remains 
brilliant  except  in  the  finest  varieties. 

Graphite  is  rare  as  a  primary  constituent  of  eruptive  rocks, 
but  if  present,  is  mostly  in  the  form  of  thick  lumps.  Altered 
limestones  and  schists  are  its  true  habitat.  It  results  in  these 
partly  by  contact-metamorphic  alteration  of  organic  matter  and 
partly  by  some  sort  of  fumarolic  action,  which  greatly  alters  the 
rock  at  the  same  time.  In  such  rocks  its  characteristic  associate 
is  rutile.  The  dust-like  variety  of  graphitoid  is  distinguished 
from  coal  to  which  it  is  very  similar  by  its  electrical  conductivity 
and  its  reaction  for  graphitic  acid.  Elementary  analysis  seems 
to  be  quite  useless  to  distinguish  between  graphite  and  coals  with 
high  fixed  carbon.  This  is  partly  on  account  of  the  organic 
impurities  consisting  chiefly  of  hydrous  silicates,  wrhich  graphite 
nearly  always  contains,  and  partly  on  account  of  an  almost 
constant  content  of  nitrogen  in  graphite.  It  is  not  even  lacking 
in  the  best  crystallized  varieties,  e.g.,  the  coarse  crystalline 
graphite  from  Ceylon. 

Carbonaceous  Matter 

Carbonaceous  matter  is  the  black  pigment  of  numerous  sedi- 
mentary rocks.  Shapeless  grains  are  found  in  the  thin  sections  of 


DESCRIPTIVE  SECTION  211 

gray  to  black  clay  slates  and  coal  slates,  and  in  gray  to  black 
limestone,  but  a  fine  opaque  dust  is  more  frequent.  This  impreg- 
nates the  whole  rock  evenly  and  makes  microscopic  analysis  of  it 
very  difficult.  Nothing  is  known  about  the  chemical  constitu- 
tion of  this  material  which  may  cause  the  black  color  of  coal. 
It  contains  oxygen,  hydrogen,  and  a  small  quantity  of  nitrogen, 
besides  carbon  evidently  in  variable  proportions.  It  must  not 
be  considered  as  established  that  a  series  of  gradual  transitions 
from  a  sedimentary  occurrence,  rich  in  oxygen,  hydrogen,  and 
nitrogen  to  a  compound  poor  in  them  exists,  as  it  was  attempted 
to  show  in  certain  phyllites  and  in  schungite  interbedded  in  layers 
of  compact  coal.  The  analyses,  which  were  designed  to  show 
that  such  was  the  case,  were  not  made  on  material  properly 
selected  but  on  very  impure  formations  and  a  certain  amount  of 
the  hydrogen  and  oxygen  reported  could  easily  be  explained  in 
this  way.  The  analyses  which  show  a  very  small  content  of 
nitrogen  are  those  of  the  purest  varieties  of  graphite.  Moreover, 
the  methods  of  determining  nitrogen  in  an  elementary  analysis 
are  not  reliable.  All  conjectures  concerning  the  chemical  prop- 
erties of  such  transition  members  are  entirely  without  foundation. 

Many  of  the  opaque  carbonaceous  substances  are  at  least 
partially  soluble  in  potassium  hydroxide  and  become  colored 
brown.  All  are  soluble  in  a  mixture  of  potassium  chlorate  and 
fuming  nitric  acid  and  give  rise  to  a  brown  liquid.  This  appears 
to  be  the  only  characteristic  reaction  to  distinguish  them  from 
graphite.  At  present  it  is  by  no  means  a  settled  question 
whether  pure  amorphous  carbon  or  a  compound  closely  related 
to  it  occurs  in  the  black  constituents  of  rocks,  which  also  appear 
as  sooty  films  on  bedding  planes  and  cleavage  faces  of  the  sedi- 
ments in  various  formations,  or  whether  the  carbonaceous 
material  is  itself  a  definite  compound  or  a  series  of  compounds, 
or  whether  or  not  they  are  definite  compounds  at  all. 

Finely  divided  black  ores  appear  very  similar  to  carbonaceous 
material.  They  can  be  distinguished  by  treating  the  slide  with 
hydrochloric  or  nitric  acid,  which  dissolve  the  ores  and  then  by 
heating  to  redness  which  destroys  the  coal.  These  processes  are 
often  necessary  before  microscopical  investigation  can  be  carried 
out  on  the  slide. 

The  dark  color  of  sediments  does  not  depend  entirely  upon  carbonaceous 
matter.  Frequently,  other  organic  compounds  with  a  brown  to  yellow 
color  and  resinous  luster  occur  in  the  sediments  along  with  carbonaceous 


212  •       PETROGRAPHIC  METHODS 

matter,  which  is  always  opaque.  These  compounds  are  called  bituminous 
matter  and  represent  a  series  of  oxidation  products  of  hydrocarbons.  Their 
constitution  is  likewise  unknown  but  they  give  to  a  rock  in  which  they 
occur  in  great  quantities  a  brown  shiny  streak.  They  are  at  least  partially 
soluble  in  benzol  or  xylol  and  are  destroyed  by  a  mixture  of  potassium 
chlorate  and  nitric  acid. 

Finally,  there  are  organic  ingredients  in  sedimentary  rocks,  which  can- 
not be  observed  microscopically  but  can  be  recognized  by  the  unpleasant 
odor,  which  they  produce  when  the  rock  is  struck — for  example,  stink 
stone.  They  are  doubtless  the  most  resistive  of  all  organic  compounds 
and  remain  unchanged  even  under  the  most  intense  metamorphic  alteration, 
which  transforms  the  other  kinds  into  graphite.  They  are  only  changed  in 
the  zone  lying  next  to  the  eruptive  contact.  Then  a  delicate  rose-red  color- 
ing matter  results  from  the  vile  smelling  substance,  which,  together  with  the 
character  of  the  odor,  indicates  that  they  are  indol  derivatives,  especially 
skatol. 

2 .  Isotropic  Minerals 

Amorphous  substances  and  cubic  crystals  are  optically  isotro- 
pic.  The  former  are  not  minerals  in  the  narrower  sense  because 
they  do  not  have  a  definite  stoichiometric  constitution.  As 
they  are  sometimes  predominant  and  sometimes  subordinate  as 
rock  constituents,  their  characteristic  properties  must  be  included 
in  this  discussion. 

Cubic  crystals  are  distinguished  from  them  by  a  regular  struc- 
ture which  is  indicated  in  the  straight  edges,  cleavage,  or  the 
arrangement  of  inclusions.  The  structure  is  sometimes  so  con- 
cealed that  it  is  not  seen  in  direct  observation  but  it  can  still  be 
determined  by  the  formation  of  etch  figures. 


FIG.  225. — Principal  Cross  Sections  of  Cubic  Minerals. 

Isotropic  minerals  are  dark  between  crossed  nicols  and  pro- 
duce no  interference  figure  in  convergent  polarized  light.  Opti- 
cal anomalies  of  various  sorts  are  very  common  and  are  recognized 
by  spotted  illumination,  by  lamination,  or  by  a  regular  division 
of  the  crystal  into  segments  that  extinguish  differently. 

Cubic  minerals  generally  develop  in  the  form  of  cubes,  octa 
hedrons,   and  dodecahedrons,   and  therefore  their  cross  sections 
are  commonly  isometric   and  three-,   four-,   or  six-sided.     The 


DESCRIPTIVE  SECTION  213 

tetragonal  trisoctahedron  is  rarer  and  its  cross  section  is  usually 
eight-sided  and  more  or  less  rounded.  Distortions  of  any  of  the 
forms  give  rise  to  variations  in  the  shape  of  the  cross  sections. 
Twins  according  to  the  spinel  law  are  not  at  all  uncommon. 
Only  the  simplest  forms  are  known  as  cleavage  fragments. 
Cleavage  parallel  to  the  cube  and  octahedron  is  frequent  and 
perfect,  while  that  parallel  to  the  dodecahedron  is  rare  and 
usually  imperfect.  Fig.  225  shows  the  forms  of  the  cross  sections 
of  minerals  in  the  cubic  system  as  well  as  the  principal  kinds  of 
cleavage. 

Perovskite  (1) 

Perovskite  occurs  as  a  rock  constituent  either  in  minute,  light  brown, 
octahedral  crystals  and  skeletons  of  crystals,  or  in  larger  compact  grains 
with  a  dark  color.  It  is  yellowish  in  reflected  light  and  has  an  adamantine 
luster.  The  smaller  crystals  are  usually  optically  normal  but  the  larger 
ones  consist  of  a  system  of  interpenetrated  double  refracting  lamellae. 
Inclusions  and  decomposition  are  not  known.  It  is  widely  disseminated  in 
basic  eruptive  rocks  associated  especially  with  melilite.  It  is  also 
known  in  certain  contact  rocks.  It  is  infusible  before  the  blowpipe.  It 
is  distinguished  from  various  minerals  that  resemble  it,  picotite,  melanite, 
etc.,  by  its  higher  index  of  refraction,  which  causes  the  high  metallic  luster 
in  reflected  light,  and  by  the  reaction  for  titanium. 

A  large  number  of  rare  cubic  minerals  appear  very  similar  to  perovskite 
under  the  microscope  with  respect  to  the  index  of  refraction,  color,  and 
geological  occurrence.  Such  are  pyrochlore,  beckelite  and  pyrrhite,  which 
crystallize  predominantly  cubical  and  are  yellowish  in  color,  also  brown 
zirkelite,  reddish  koppite,  knopite,  which  usually  shows  optical  anomalies, 
and  dysanalyte  with  its  metallic  luster,  being  almost  opaque  in  thin  section. 
All  these  minerals  are  local  constituents  of  eruptive  rocks  and  pegmatites 
rich  in  soda,  but  are  found  also  in  grains  in  the  contact  rocks  lying  next  to 
these.  Oldhamite,  CaS,  also  occurs  in  meteorites  but  it  is  not  observed 
except  in  specially  prepared  sections  because  it  decomposes  in  -water. 

Sphalerite  (1) 

Sphalerite  is  a  widespread  constituent  of  rocks,  occurring  in  irregular 
grains  associated  with  opaque  sulphides.  It  is  light  yellow  to  brownish, 
and  appears  very  much  like  rutile  and  was  almost  always  determined  as 
such  until  recently.  It  is  distinguished  from  rutile  by  a  more  perfect  cleav- 
age and  its  isotropic  behavior  toward  light. 

Sphalerite  may  also  be  confused  with  the  rare  isotropic  minerals  mentioned 
under  perovskite  with  which  it  is  associated  in  contact  rocks,  especially  in 
limestone.  It  can  only  be  determined  positively  by  a  blowpipe.  It  gives  a 
film  of  zinc  oxide  when  fused  with  sodium  carbonate  on  charcoal. 


214 


PETROGRAPHIC  METHODS 


Garnet  Group  (1) 

The  minerals  of  the  garnet  group  are  widespread  rock  constit- 
uents. They  are  sometimes  well  developed  crystals,  and  some- 
times rounded  or  irregular  grains.  The  most  frequent  crystal 
form  is  the  rhombic  dodecahedron,  Fig.  226,  and  the  tetragonal 
trisoctahedron,  Fig.  227,  is  less  common.  Combinations  of  these 
two  forms,  Fig.  228,  also  occur.  Their  dimensions  vary  greatly 
and  every  gradation  from  microlites  to  crystals  as  large  as  a  fist 
are  found.  Individuals  of  intermediate  sizes  are  most  common 
and  they  are  macroscopically  visible.  Almandite  and  the  lime 
garnets  often  form  perimorphs.  Alterations  that  are  compara- 
tively rare,  in  which  chlorite,  amphibole,  or  biotite  are  formed  at 
the  expense  of  the  garnet,  appear  similar  to  these. 


Fia.  226.  FIG.  227.  FIG.  228. 

Dodecahedron.          Tetragonal  Tris-  Combination. 

octahedron. 


Cleavage  parallel  to  the  dodecahedron  is  at  most  only  suggested, 
Fig.  225,  e  and  /.  But  on  account  of  its  brittleness  the  garnet 
crystals  are  usually  quite  full  of  short  jagged  cracks  that  do  not 
indicate  any  crystallographic  orientation. 

Optical  anomalies  are  common  but  are,  confined  mostly  to  the 
group  of  lime  garnets  free  from  titanium  and  to  the  manganese 
garnets.  This  consists  of  a  division  of  the  crystal  into  segments, 
that  are  optically  different,  and  often  also  of  a  zonal  structure 
due  to  the  growth  of  layers  with  very  strong  but  variable  double 
refraction.  The  division  into  segments  always  corresponds  to 
one  of  the  crystal  forms  present.  The  crystal  then  appears  to  be 
composed  of  as  many  pyramids  as  there  are  faces  to  that  form  and 
each  face  is  the  base  of  a  pyramid  whose  apex  lies  in  the  center  of 
the  crystal.  The  double  refraction  of  these  segments  is  quite 
variable  and  may  exceed  that  of  quartz.  They  are  biaxial, 
optically  positive  often  with  strong  dispersion,  p<  v}  and  2v  =  56° 
to  90°.  Anomalous  interference  figures  are  frequent.  Each  one 
of  these  segments  is  usually  not  very  homogeneous  but  is  built  up 


DESCRIPTIVE  SECTION 


215 


of  differently  orientated  parts  that  are  more  or  less  irregularly 
bounded,  so  that  the  character  of  the  optical  anomalies  stands 
out  prominently  and  can  readily  be  distinguished  from  normal 
double  refracting  substances. 

The  dodecahedron  is  the  most  frequent  form  of  garnet  crystals 
and,  therefore,  the  optical  structure  is  usually  the  so-called 
dodecahedral  structure.  Fig.  229  shows  a  section  parallel  to  the 
dodecahedron  face  and  Fig.  230  shows  one  out  of  the  middle  of  a 
crystal  parallel  to  an  octahedron  face.  A  similar  development 
occurs  now  and  then  in  almandite  unaccompanied  by  optical 
anomalies,  and  is  due  to  the  arrangement  of  inclusions,  or  to 
breaking  parallel  to  the  pyramids  of  growth. 

Almandite  is  the  most  widespread  member  of  the  garnet  group. 
The  larger  crystals  are  macroscopically  transparent  and  wine  red, 
and  are  termed  precious  garnet,  while  the  more  frequent  red  to 


FIG.  229. — Segments  in  Garnet. 


FIG.  230. — Garnet  with  Optical  Anomalies. 
Section  Parallel  to  Octahedron. 


yellowish-red  varieties  clouded  by  inclusions  or  cracks,  are  called 
common  garnet.  Both  varieties  are  pale  reddish  in  thin  section. 
Almandite  is  always  isotropic.  It  is  an  accessory  constituent  of 
all  eruptive  rocks  with  the  exception  of  the  most  basic  ones.  It 
is  characteristic  for  granulite.  It  is  more  common  and  frequent 
as  an  essential  constituent  of  various  groups  of  contact  rocks.  It 
is  usually  macroscopically  visible.  Concentric  structures  often 
develop  around  almandite  and  these  tend  to  extend  outward 
from  a  compact  grain  into  the  surrounding  minerals  like  pseudo- 
pods.  The  larger,  well  defined  crystals  are  easily  removed  from 
rocks  having  abundant  mica,  while  they  are  apt  to  be  broken  in 
rocks  with  little  mica. 

The  mineral  is  sometimes  altered  to  chlorite  or  hornblende, 
and  in  aplites  and  pegmatites,  in  which  almandite  is  the  most 
characteristic  accessory  constituent,  round  flakes  of  biotite  are 
seen  which  contain  a  residue  of  garnet  from  which  the  biotite 


216 


PETROGRAPHIC  METHODS 


was  formed.  Almandite  is  quite  resistive  to  weathering  and  is, 
therefore,  often  found  in  secondary  deposits.  It  is  used  as  a 
substitute  for  emery.  A  very  fine  skeletal  development  has  been 
observed  in  clastic  rocks  but  nothing  more  is  known  about  this 
formation.  Almandite  has  the  greatest  tendency  to  crystalliza- 
tion of  all  rock-forming  minerals  and  is,  therefore,  frequently 
filled  with  inclusions.  Almandite  crystals,  perforated  like  a 
sieve,  are  the  most  characteristic  features  of  contact  rocks.  In 
rocks  rich  in. quartz  the  garnet  itself  takes  a  small  part  in  the 
composition  of  such  a  structure,  Fig.  214,  page  197.  It  also  oc- 
curs in  the  most  beautiful  perimorphs  in  such  rocks,  Fig.  213, 
page  197. 

The  manganese  garnet,  spessartite,  is  very  similar  to  almandite 
in  its  occurrence  as  a  rock  constituent  but  is  much  rarer.  It  is 
common,  especially  in  pegmatite  dikes.  It  forms  also  a  principal 

constituent  of  the  whetstone 
schists  of  the  Ardennes  moun- 
tains but  occurs  in  minute 
colorless  individuals. 

Pyrope,  usually  containing 
some  chromium,  is  confined  to 
the  olivine  ,rocks  and  their 
derivatives,  the  serpentines. 
It  generally  shows  rather  in- 
distinct crystal  form,  which 
appears  to  be  hexahedral.  Its 
individuals  can  be  recognized 
distinctly  macroscopically  by 
the  blood  red  color.  It  is  dis- 
tinguished from  almandite  by 
being  poor  in  inclusions  and 
cracks  and,  therefore,  often  transparent — cinnamon-stone.  The 
occurrence  of  a  fine  fibrous,  radial  border  of  kelyphite  is  especially 
distinctive,  Fig.  231.  It  sometimes  consists  of  hornblende 
minerals  but  is  more  often  a  mixture.  It  often  completely 
replaces  the  pyrope.  This  border  is  considered  a  product  of 
magmatic  resorption  of  pyrope,  that  has  already  been  formed,  by 
a  magma  rich  in  magnesium.  Pyrope  is  also  found  in  secondary 
deposits  in  comparatively  large  grains,  which  are  cut  as  semipre- 
cious stones — Bohemian  garnet. 

Hessonite,  rich  in  iron,  and  topazolite  of  the  lime  garnet  series 


FIG.  231.— Pyrope  with  Kelyphite  Border 
in  Serpentine  from  Karlstetten,  Lower 
Austria.  (After  Cohen.) 


DESCRIPTIVE  SECTION  217 

are  found  almost  exclusively  in  crevices  and  other  forms  of 
secondary  development  within  rocks,  especially  in  serpentine. 
Light  green  topazolite,  demantoid,  is  sometimes  found  in  dis- 
seminated crystals.  These  minerals  are  also  the  most  important 
constituents  of  skarn,  which  accompanies  magnetite  ore  deposits. 
Emerald  green  uwarowite,  an  optically  anomalous  lime  garnet 
containing  chromium,  is  likewise  confined  to  crevices  in  serpen- 
tine. Grossularite  and  hessonite  poor  in  iron  are  widely  dis- 
seminated constituents  of  contact-metamorphic  limestones  and 
lime-silicate  hornfels.  The  former  is  entirely  colorless  in  thin 
section,  and  the  latter  is  light  reddish,  brownish,  or  greenish,  the 
color  being  entirely  independent  of  the  chemical  composition. 
Light  reddish,  excellently  bounded  microlites  of  lime  garnet 
often  fill  the  plagioclase  crystals  of 'the  granite  of  the  Central  Alps 
in  such  a  manner  that  the  rock  has  a  reddish  tinge.  When  these 
garnets  occur  in  limestones,  they  have  a  crystallographic  form 
but  are  often  rounded  in  a  peculiar  manner  as  though  they  had 
been  slightly  fused  on  the  surface.  In  other  rocks  they  form 
granular  aggregates  without  distinct  crystal  form.  Lime  gar- 
nets poor  in  iron  are  easily  fusible,  and  those  rich  in  iron  fuse 
with  difficulty. 

The  lime  garnets  have  fewer  inclusions  than  almandite,  but 
perimorphs  are  encountered  in  them.  Evidences  of  alteration 
are  almost  entirely  lacking.  On  the  other  hand,  a  structure  con- 
sisting of  different  zones  is  very  common. 

The  members  of  the  lime  garnets  containing  titanium  play  a 
special  r61e.  They  are  macroscopically  black  with  a  pitchy 
luster  but  in  thin  section  are  brown  and  built  up  of  zones  in  which 
the  intensity  of  the  color  varies  depending  upon  the  variations  in 
the  content  of  titanium.  These  garnets  occur  especially  in 
eruptive  rocks  rich  in  sodium  and  their  contact  formations,  and 
are  usually  well  developed  and  always  poor  in  inclusions.  They 
are  named  according  to  the  increasing  content  of  titanium — 
melanite,  schorlomite  and  ivaarite,  the  latter  containing  about 
15  per  cent.  TiO2.  The  cause  of  the  black  color  of  pyrenaite, 
which  occurs  in  the  contact  rocks  of  the  Pyrenees,  is  to  be  sought 
not  in  these  garnets  but  in  finely  divided  graphite. 

The  silicates  almandite,  pyrope,  and  hessonite  form  isomor- 
phous  mixtures  only  in  exceptional  instances.  The  garnet  of 
eclogites,  which  corresponds  apparently  to  normal  almandite,  is 
such  a  mixture  in  which  almandite  predominates.  Dense  splin- 


218  PETROGRAPHIC  METHODS 

tery  aggregates  of  grossularite  are  noteworthy  in  olivine  rocks  and 
serpentine.  They  occur  in  lumps  or  form  a  part  of  the  so-called 
saussurite.  Similar  formations  of  hessonite  occur  on  the  con- 
tact of  dikes  containing  hessonite,  with  olivine  or  serpentine 
rocks.  The  latter  have  been  shown  to  be  alteration  products  of 
the  olivine  rock  itself. 

Almandite  and  pyrope  are  not  easily  confused  with  other 
minerals  because  of  the  reddish  tint  which  is  always  plainly  seen 
even  in  the  thinnest  sections.  Colorless  or  greenish  lime  garnets 
resemble  the  spinels,  especially  when  they  are  in  small  individ- 
uals, and  when  the  crystal  form  is  lacking  they  can  only  be 
distinguished  by  a  chemical  test.  They  can  be  differentiated 
from  periclase,  which  they  likewise  simulate,  by  the  imperfect- 
ness  of  the  cleavage.  Titanium  garnets  may  be  confused  with 
picotite  or  perovskite,  but  the  garnet  usually  has  the  zonal 
structure.  Chemical  reactions  will  give  the  necessary  confirma- 
tion. Double  refracting  lime  garnets  are  so  similar  to  vesuvianite 
and  to  the  low  double  refracting  members  of  the  epidote  group, 
especially  when  the  garnet  is  in  fine  grained  aggregates,  that  a 
differentiation  is  not  possible. 

Spinel  Group  (1) 

The  minerals  of  the  spinel  group  (compare  magnetite  and  chromite,  page 
206)  occur  in  sharp  octahedral  crystals  and  twins  according  to  the  spinel 
law,  Figs.  221  and  222,  page  206,  without  cleavage,  and  less  often  in  irregular 
grains.  They  are  always  optically  normal.  They  are  the  hardest  and 
most  resistive  rock-forming  constituents  and  are,  therefore,  scarcely  ever 
altered  and  are  frequently  found  in  secondary  deposits. 

Chrome  spinel,  picotite,  forms  minute  crystals  nearly  everywhere  in 
olivine  with  which  it,  like  chromite,  is  always  associated.  It  often  shows 
no  crystal  form  when  it  occurs  as  an  independent  constituent  of  peridotite. 
It  is  macroscopically  black  with  a  submetallic  luster,  but  under  the  micro- 
scope the  metallic  luster  is  not  seen.  It  is  brown  in  transmitted  light  but 
usually  lighter  than  chromite  to  which  it  is  exactly  similar  except  for  the 
hardness.  The  iron  spinels,  pleonaste  and  hercynite,  are  black  as  is  also  the 
zinc  spinel,  kreittonite,  which  occurs  in  certain  ore  deposits  and  is  very 
similar  to  the  others,  being  distinguishable  from  them  only  chemically. 
They  have  dull  luster,  green  streak,  and  are  green  and  transparent  in  thin 
section.  Common  spinel  is  red,  green,  blue  or  violet  to  the  naked  eye,  but 
is  always  colorless  in  thin  section.  It  is  a  characteristic  product  of  contact 
metamorphism  particularly  in  granular  limestones  and  dolomites.  It  can 
be  recognized  macroscopically,  and  on  account  of  its  dearth  of  inclusions  and 
lack  of  cleavage,  it  is  clear  and  transparent — precious  spinel.  Iron  spinel 
is  also  found  in  other  rocks,  such  as  granulites  and  Iherzolites.  It  is  mostly 


DESCRIPTIVE  SECTION  219 

in  the  outer  border  zone  of  the  eruptive  rock  in  schlieren  and  is  often  mixed 
with  other  contact  minerals.  It  is  undoubtedly  a  residue  of  resorbed  in- 
clusions of  the  neighboring  rock.  The  minutest  microlites  of  colorless 
spinel  are  found  in  the  so-called  fritted  rocks  and  only  the  high  index  of 
refraction  serves  to  determine  it.  Spinel  can  be  easily  isolated  in  all  cases 
on  account  of  its  resistance  to  reagents.  The  distinguishing  of  the  spinels 
from  perovskite  and  garnet  is  discussed  under  those  minerals.  The  lack  of 
cleavage  and  its  chemical  behavior  distinguish  it  from  periclase. 

Periclase  (1) 

Periclase  is  not  a  rare  constituent  of  contact-metamorphic  limestones. 
It  is  usually  in  small  octahedrons  with  rounded  surfaces.  They  are  macro- 
scopically  colorless  to  greenish-brown.  When  fresh  it  is  easily  recognized 
by  the  cleavage,  Fig.  225,  c,  page  212,  and  the  index  of  refraction.  It  is  usu- 
ally strongly  altered  to  scaly  aggregates  of  brucite,  e.g.,  in  predazz.ite, 
or  of  serpentine.  In  these  the  original  cleavage  is  sometimes  retained. 

Boracite  (1) 

Boracite  is  a  rock-forming  mineral  now  and  then,  but  is  confined  to  the 
rock-salt  formations.  It  occurs  in  gypsum  and  abraum  salts  in  well 
developed  tetrahedral  crystals  that  are  macroscopically  recognizable. 
They  are  colorless  to  light  green,  often  possessing  an  abundance  of  faces. 
Under  the  microscope  it  appears  very  much  like  leucite  from  which  it  is 
distinguished  by  its  higher  index  of  refraction  and  its  mode  of  occurrence. 

Rock  Salt 

Rock  salt  as  a  rock  constituent  is  not  observed  in  thin  sections  prepared 
with  water  and  is,  therefore,  not  included  in  the  tables.  H.  =  2;  sp.  gr.  =2.2. 
Perfect  cubical  cleavage,  Fig.  220,  page  206.  It  is  without  crystal 
form  and  is  soluble  in  water  giving  a  salty  taste,  n  =  1.544.  It  often  occurs 
in  pure,  very  coarse  granular  aggregates  that  are  colorless,  deep  blue,  red 
or  yellow,  and  rich  in  liquid  inclusions,  or  in  fibrous  masses  in  crevices,  or 
finally  in  fine  impregnations  in  salt  clays.  It  also  occurs  as  incrustations 
on  lava  rocks  and  as  small  cubes  in  liquid  inclusions  in  quartz.  ~- 

Leucite  (1) 

Leucite  is  found  almost  exclusively  in  white,  perfectly 
developed  tetragonal  trisoctahedrons,  Fig.  227,  page  214,  in 
eruptive  rocks  rich  in  alkali,  especially  the  leucitophyres,  leucite 
tephrites,  etc.  In  these  rocks  it  frequently  forms  large,  brittle, 
and  checked  crystals,  but  in  leucite  basalt  it  is  only  in  the  form 
of  microlites.  Aggregates  of  orthoclase  and  nepheline  in  the 
form  of  large  leucite  crystals  are  found  in  leucite  syenites  and  the 
rocks  produced  by  differentiation  of  such  a  magma. 


220  PETROGRAPHIC  METHODS 

The  outlines  are  usually  rounded.  Skeletal  development  and 
magmatic  resorption  are  not  rare.  Small  crystals  are  always 
optically  normal  but  larger  crystals  consist  of  complicated 
lamellar  twinning  intergrowths  and  penetration  of  such  a  charac- 
ter that  a  highly  characteristic  structure  is  observed  in  polarized 
light,  as  is  shown  in  Fig.  232.  When  heated  to  600°  this  anoma- 
lous structure  disappears  but  returns  again  upon  cooling. 

The  frequency  of  inclusions  is  an  especially  characteristic 
property  of  leucite.  They  consist  of  crystallized  minerals,  slag, 
or  colorless  to  brownish  glass,  arranged  in 
a  regular  manner  either  zonal  or  radial,  Fig. 
210,  page  195.  Needles  of  various  minerals, 
arranged  tangentially  around  the  crystals  of 
leucite,  are  often  observed,  ocellary  struc- 
ture, Fig.  207,  page  193.  It  is  often  altered 
because  it  is  so  readily  attacked.  Besides 
the  pseudomorphs  referred  to  above  those 
FiG.232:-TwinningLam-  of  analcite  and  other  zeolites  are  wide- 

ination  in  Leucite. 

spread. 

It  would  be  very  easily  overlooked  where  it  occurs  in  the  form 
of  microlites,  but  even  they  are  distinguished  from  all  other 
minerals  by  the  arrangement  of  the  inclusions.  It  could  be 
confused  with  microcline  when  it  forms  compact  aggregates  that 
are  optically  anomalous,  but  its  mode  of  occurrence  is  entirely 
different.  If  the  optical  anomalies,  the  crystal  form,  and  the 
inclusions  are  lacking,  distinction  from  analcite,  sodalite,  or  from 
rock  glass  is  scarcely  possible. 

Glass  (1) 

Glassy  material  is  found  as  a  principal  or  subordinate  rock  constituent 
only  in  rocks  which  have  solidified  rapidly  from  the  magma.  These  are 
the  comparatively  small  eruptive  dikes,  which  have  branched  out  far  from 
the  vulcanic  center,  or  the  rocks  poured  out  upon  the  surface  and  their  tuffs. 
or  those  rocks  or  fragments  which  have  suffered  partial  fusion,  i.e.,  fritting. 
The  glass  occurring  in  rocks  may  have  quite  a  variable  composition,  but  the 
acid  mixtures  tend  in  general  more  to  glassy  development  than  do  the  more 
basic  ones.  Although  in  andesite,  diabase  and  basalt,  glasses  are  not 
lacking,  still  the  series  of  quartz  porphyry,  rhyolite,  and  trachyte  is  much 
richer  in  rock  glass. 

Some  of  the  glasses  are  free  from  water  while  others  show  an  original 
content  of  water  up  to  8  or  10  per  cent.  The  former  are  called  obsidian  and 
the  latter  pitchstone,  while  perlite  with  2.5  to  5  per  cent,  of  water  is  inter- 


DESCRIPTIVE  SECTION 


221 


mediate.  The  specific  gravity  is  always  low,  rising  to  2.4  in  obsidian  and 
2.7  in  basic  glasses,  and  falling  to  2.25  in  those  containing  water. 

The  normal  rock  glasses  have  a  chemical  composition  corresponding  to 
that  of  crystalline  rocks.  They  are  principally  products  of  rapid  solidi- 
fication of  granitic  rock  magmas,  but  glasses  with  the  composition  of  ande- 
site  and  even  trap  and  basalt  are  not  altogether  lacking  although  they  are 
in  general  anomalous  forms  of  development.  It  has  been  noted  that 
partially  or  entirely  resorbed  inclusions  have  given  rise  to  the  formation  of 
glass  because  the  ability  of  the  dissolving  magma  to  crystallize  has  been 
lost  through  the  chemical  composition  of  the  inclusions.  The  glassy 
development  of  basic  rocks  may  also  be  referred  to  such  phenomena.  These 
glassy  forms  occur  sometimes  in  rounded  masses  within  normal  rocks  or  on 
the  contact  with  certain  rocks,  while  they  are  entirely  lacking  with  other 
rocks.  Chemical  analysis  almost  always  shows  variations  from  the 
normal  composition  in  such  cases. 

The  chemical  behavior  of  rock  glasses  is  quite  variable  according  to  the 
varying  composition.  In  general,  the  glasses  .are  more  rapidly  attacked  by 
the  atmosphere  and  are  more  easily  dissolved  by  hydrofluoric  acid  than 
are  most  of  the  crystallized  rock  constituents.  Most  of  the  rock  glasses 
are  very  little  affected  by  hydrochloric 
acid,  but  now  and  then  they  gelatinize 
with  hydrochloric  acid,  especially 
basaltic  glasses  rich  in  sodium.  Rock 
glasses  often  appear  to  have  double 
refraction  in  the  neighborhood  of 
inclusions.  This  is  produced  by  strain 
and  gives  rise  to  the  Brewster's  cross, 
Part  I,  page  123.  Obsidian  some- 
times forms  the  predominant  constit- 
uent of  rocks  and  in  them  only  a 
few  devitrification  products  occur. 
It  is  brittle  and  has  a  perfect  con- 
choidal  fracture,  Fig.  233,  vitreous 
luster,  and  is  almost  invariably  dark 
grayish-black  to  pitch  black  in  color. 
In  thin  section  the  color  can  often  be 
plainly  recognized,  but  it  is  sometimes 
entirely  colorless  and  frequently  filled 
full  of  trichites.  Upon  heating  many 

varieties  swell  out  into  foamy  masses,  which  are  light  gray  to  white  and 
are  called  pumice.  Pumice  in  the  form  of  ejectamenta  and  bombs  is 
often  found  in  nature  accompanying  obsidian.  The  obsidians  belonging  to 
basic  rocks,  such  as  the  bluish-black  tachylite  and  the  black  hyalomelane, 
are  heavy  and  have  comparatively  high  indices  of  refraction.  They  are 
quite  highly  colored  in  thin  section. 

Perlite  is  a  rock  glass,  which  is  full  of  curved  cracks  throughout  its  mass, 
so  that  it  breaks  up  easily  into  rounded  fragments  each  of  which  has  an 
onion-like  structure.  It  usually  has  a  lower  specific  gravity  than  obsidian 
because  of  these  cracks,  and  is  lighter  colored  macroscopically.  Under 


FIG.  233. — Conchoidal  Fracture    (1/2 
Natural   Size).     Obsidian,  Iceland. 


222 


PETROGRAPHIC  METHODS 


the  microscope  the  onion-like  arrangement  of  the  cracks  presents  an  un- 
commonly characteristic  appearance,  Fig.  234.  Pitchstone  has  a  pitchy 
luster  and  is  variously  colored  macroscopically.  It  may  be  green,  yellow, 
red,  or  black.  The  pitchy  luster  may  be  due  to  an  abundant  development 
of  microlitic  crystallization.  Crystal  skeletons  assume  manifold  forms  in 
pitchstone. 

Rock  glasses  often  possess  the  appearance  of  decided  flow  structure.  In 
these  glasses  variously  colored  bands  mixed  with  each  other  in  multifarious 
ways  form  the  principal  constituent.  In  other  cases  they  flow  around  the 
large  crystals  that  have  separated  out,  Fig.  235,  eutaxite,  so  that  it  appears 
as  if  the  different  parts  were  not  miscible  with  each  other  even  in  the  liquid 
condition.  Alteration  of  beds  greatly  devitrified  with  those  less  so,  and 
of  porous  with  denser  layers,  is  not  rare  in  rock  glasses.  The  phenomenon  of 
devitrification  is  to  be  especially  emphasized.  The  original  glass,  which  is 


FIG.  234.— Perlite,  Glashiittental  near 
Schemnitz,  Hungary. 


FIG.  235. — Eutaxite.  Microscopic  Fluidal 
Structure  in  Pitchstone.  Kastelruth,  South 
Tyrole. 


completely  amorphous,  is  transformed  into  a  crystalline  aggregate  called 
microfelsite,  felsite,  etc.  The  newly  formed  mineral  aggregates — structure- 
less combinations  of  quartz  and  orthoclase  in  the  ordinary  acid  glasses, 
are  considered  to  be  the  result  of  a  long  period  of  molecular  rearrangement 
somewhat  analogous  to  the  transformation  of  the  metals.  The  cause  has 
also  been  sought  in  circulating  waters,  either  of  a  meteoric  or  of  magmatic 
character. 

When  the  glass  occurs  in  the  form  of  fine  films  as  a  subordinate  base 
between  the  crystalline  constituents  and  is  color'ess,  it  is  very  hard  to 
determine.  It  can  scarcely  be  distinguished  from  nepheline  or  from  zeolites, 
especially  in  those  cases  where  a  feeble  double  refraction  is  shown  on  account 
of  strain.  If  it  forms  the  residue  from  which  the  other  constituents  have 
crystallized,  it  is  always  richer  in  silica  than  the  normal  rock. 

Analcite  (1) 

Analcite  is  only  known  as  a  secondary  rock  constituent  in  druses  and  in 
pseudomorphs.  It  occurs  in  various  types  of  soda  rocks  and  is  usually 


DESCRIPTIVE  SECTION  223 

formed  from  sodalite,  nepheline,  and  leucite.  As  a  rock  constituent  it 
does  not  have  its  own  crystal  form.  Crystals  are  only  found  in  druses  and 
cavities.  Optical  anomalies  are  present  now  and  then,  but  they  are  mostly 
much  weaker  than  in  garnet.  Analcite  usually  appears  clear  and  trans- 
parent. Sometimes  the  cleavage  cracks  are  quite  plain,  but  frequently  it 
does  not  possess  these  and  then  the  determination  is  made  in  a  chemical 
manner  by  testing  for  sodium.  The  double  refraction  also  increases  and 
it  becomes  clouded  when  heated.  The  mineral  itself  has,  however,  no  char- 
acteristic microstructure  and  no  striking  properties.  It  is  most  easily  con- 
fused with  leucite,  sodalite,  and  nepheline. 

Sodalite  Group  (2) 

The  usual  crystal  form  of  the  minerals  of  the  sodalite  group  is 
the  rhombic  dodecahedron,  Fig.  226,  page  214.  The  cross  sec- 
tions are  therefore  six-sided  or  quadratic,  and  often  somewhat 
distorted.  The  edges  are  usually  'rounded  and  the  faces  are 
eaten  out  and  corroded.  Sodalite  is  found  now  and  then  in 
irregular  grains,  which  is  not  the  case  with  the  other  members  of 
the  group.  The  dimensions  are  usually  not  very  small  and 
microlites  are  entirely  lacking.  These  minerals  are  easily 
recognized  by  the  naked  eye  when  they  are  colored.  Colorless 
fresh  varieties  may  be  easily  overlooked,  but  they  show  plainly 
on  the  fracture  surface  of  the  rock  if  they  are  clouded  by  incipient 
alteration.  They  then  appear  dull  white  or  yellowish  and  are 
easily  confused  with  altered  feldspar  from  which  they  can  be 
distinguished  by  the  lack  of  cleavage  and  the  typical  isometric 
form  of  the  cross  section. 

The  sodalite  minerals  are  found  in  rather  basic  soda  rocks  free 
from  primary  quartz  and  constitute  the  part  most  susceptible 
to  alteration.  They  are  frequently  altered  to  natrolite,  analcite, 
and  other  zeolites,  forming  aggregates  like  snowflakes — Spreu- 
stein.  Alteration  to  dense  aggregates  of  mica  or  to  amorphous 
masses  is  also  known. 

Sodalite  is  disseminated  throughout  certain  granular  eruptive 
rocks  and  is  macroscopically  colorless,  or  at  most  very  light  blue 
or  green,  but  in  thin  section  it  is  always  colorless.  Haiiyne  is 
more  often  found  in  extrusive  rocks  and  is  sometimes  colorless, 
while  at  other  times  it  is  gray,  yellow,  green,  red,  or  deep  blue. 
It  is  frequently  speckled.  Certain  colorless  occurrences  have 
the  property  of  taking  on  an  intense  blue  color  when  heated. 
If  such  varieties  are  treated  with  hydrofluoric  acid  and  silver 
nitrate  they  percipitate  black  silver  sulphide,  while  sodalite 


224 


PETROGRAPHIC  METHODS 


FIG.  236. — Haiiyne  with 
Regular  Inclusions. 


becomes  coated  with  silver  chloride.  Regularly  arranged  black 
bars  with  unknown  properties  are  frequently  observed  in  soda- 
lite  minerals.  They  have  a  tendency  to  congregate  on  the  edge 
of  the  crystals  and  make  them  less  transparent,  Fig.  236.  Such 
borders,  which  are  quite  small  but  entirely  opaque,  are  observed 
especially  around  crystals,  which  have  been 
greatly  rounded  and  corroded  by  the  molten 
magma.  Ferric  hydrate  forms  from  these 
dark  inclusions  when  the  rock  is  weathered, 
and  it  colors  the  mineral  yellowish  to  red. 
Other  inclusions,  ores,  pyroxene,  etc.,  are 
frequent,  but  glass  is  comparatively  rare. 
If  the  crystal  form  is  lacking,  the  regularly 
arranged  inclusions  together  with  the  low 
index  of  refraction  is  often  the  only 

characteristic  feature  because  they  show  very  imperfect  cleavage, 
which  is  at  best  only  irregular  cracks. 

Differentiation  of  the  various  members  is  only  possible 
chemically.  The  test  for  chlorine  in  sodalite  has  already  been 
mentioned.  If  haiiyne  is  treated  with  hydrochloric  acid,  gypsum 
crystallizes  out.  The  same  crystals  are  obtained  from  noselite  if 
a  trace  of  calcium  carbonate  is  added  to  the  solution.  The  index 
of  refraction  being  lower  than  that  of  Canada  balsam  distin- 
guishes them  from  other  similarly  appearing  minerals,  but  that 
may  give  rise  to  confusion  with  zeolites,  tridymite,  or  rock 
glass.  They  can,  however,  be  positively  differentiated  from 
analcite  and  glass  only  by  special  chemical  reactions. 

Lazurite  may  be  referred  to  briefly.  It  occurs  in  irregular  grains  forming 
the  deep  blue  coloring  constituent  of  lapis  lazuli.  It  is  a  member  of  the 
sodalite  group  containing  sodium  sulphide  and  causes  the  blue  color  of  the 
other  members.  Lapis  lazuli  is  a  contact  rock  containing  also  calcium- 
magnesium  silicates  and  carbonates. 

Opal  (2) 

Opal,  as  a  rock  constituent,  is  not  often  visible  macroscopically.  It 
may  be  developed  as  precious  opal  with  the  characteristic  p]ay  of  colors 
or  as  dull  colored  common  opal  clouded  by  numerous  inclusions.  It  is 
always  secondary,  filling  crevices,  or  occurring  in  pseudomorphs  after 
feldspar  and  other  minerals.  It  is  found  especially  in  greatly  altered  erup- 
tive rocks  and  their  tuffs.  It  is  not  easy  to  ascertain  to  what  extent  opal 
occurs  as  a  constituent  of  sedimentary  rocks.  The  amorphous  opal-like 
silica  of  organisms  is  very  easily  transformed  into  crystalline  aggregates 
and,  on  the  other  hand,  opal  sandstones  occur  in  such  relationships  that 


DESCRIPTIVE  SECTION 


225 


the  formation  of  the  mineral  from  thermal  processes  is  highly  probable.  In 
rocks  in  which  opal  is  not  seen  macroscopically  it  is  observed  in  thin  sec- 
tions in  finely  divided,  shapeless,  and  colorless  impregnations  which  may  be 
recognized  only  with  great  difficulty.  This  type  of  occurrence  also  shows 
all  the  earmarks  of  post  volcanic  alteration.  It  is  always  colorless  under  the 
microscope  and  may  be  recognized  by  its  very  low  index  of  refraction.  It 
often  shows  anomalous  double  refraction,  Brewster's  cross,  and  inclusions 
of  tabular  crystals  of  tridymite  are  extremely  common.  A  microscopic 
differentiation  from  glass  is  often  very  difficult,  although  the  low  index  of 
refraction  is  very  distinctive.  It  can  be  determined  positively  by  its  solu- 
bility in  potassium  hydroxide.  Even  this  reaction  may  give  rise  to  error 
because  numerous  decomposition  products  containing  aluminium  also  yield 
silica  in  potassium  hydroxide. 

Fluorite  (2) 

Fluorite  is  rare  as  a  real  constituent  of  rocks.  It  is  found  in  granite, 
especially  in  the  facies  altered  by  pneumatolytic  processes — greisen — 
together  with  tourmaline,  topaz,  etc.  It  is  also  found  as  a  constant  asso- 
ciate of  the  zirconium  silicates,  that  occur  accessory  in  soda  rocks.  It  is 
always  in  grains  and  is  easily  recognized  when  it  is  violet,  but  very  difficult 
to  find  when  it  is  colorless.  It  is  characterized  by  the  fact  that  it  has  the 
lowest  index  of  the  isotropic  rock-forming  minerals  and  also  has  a  perfect 
cleavage,  which  usually  causes  a  scaly  appearance  to  the  surface  of  the 
section. 

3.  Uniaxial  Minerals 

The  rock-forming  minerals  of  the  tetragonal  and  hexagonal 
systems  are  sometimes  long,  sometimes  short  prismatic,  with 


FIG.  237. — Principal  Cross  Sections  of  Uniaxial  Minerals. 

or  without  pyramidal  terminations.  Sometimes,  they  are  devel- 
oped tabular  parallel  to  the  base  or  are  predominantly  pyramidal 
or  rhombohedral.  Fig.  237  shows  the  most  important  types  of 

15 


226  PETROGRAPHIC  METHODS 

cross  sections.  In  the  prism  zone  they  are  either  elongated  or 
short,  almost  square  rectangles  with  or  without  domatic  forms 
on  the  long  or  the  short  side.  Rhombic  forms,  with  or  without 
truncated  corners,  are  also  observed.  In  the  first  case  the 
extinction  is  always  parallel  and  perpendicular  to  the  sides  of 
the  rectangle,  while  in  the  latter  case  the  extinction  direction 
bisects  the  angles  of  the  rhombus. 

A  section  orientated  as  nearly  parallel  to  the  optic  axis  as 
possible  is  found  by  choosing  the  one  which,  in  parallel  polar- 
ized light,  shows  the  highest  interference  color  of  all  the  sections 
of  the  same  mineral.  This  is  also  an  indication  of  the  strength 
of  the  double  refraction.  In  convergent  polarized  light  such  a 
section  shows  a  distribution  of  colors  into  curves  simulating 
hyperbolae.  They  are  symmetrical  with  respect  to  two  planes, 
and  the  color  is  lowered  in  those  quadrants  through  which  the 
optic  axis  passes  and  is  raised  in  the  other  two  quadrants  with 
respect  to  the  center  of  the  field.  The  optical  character  of  the 
principal  zone,  Ch2,  is  the  same  as  that  of  the  mineral  itself, 
Chm,  because  in  uniaxial  minerals  with  a  prismatic  development 
the  optic  axis  and  the  axis  of  the  principal  zone  coincide.  If 
uniaxial  minerals  are  tabular  parallel  to  the  base,  the  optic  axis 
is  perpendicular  to  the  principal  zone,  and  the  character  of  the 
principal  zone  is  opposite  to  that  of  the  mineral. 

Sections  cut  perpendicular  to  the  optic  axis  remain  dark  when 
rotated  between  crossed  nicols  in  parallel  light  and  appear  like 
isotropic  minerals,  but  if  the  double  refraction  is  not  too  small,  as 
it  is  in  apatite,  such  sections  give  an  interference  figure  in  con- 
vergent light.  Their  outlines  are  quadratic,  or  rarely  eight- 
sided  in  tetragonal  minerals,  and  six-  or  three-sided  in  hexagonal 
minerals.  Occasionally  where  trigonal  forms  occur  the  sections 
are  nine-sided.  Such  sections  give  the  interference  figure  of  uni- 
axial crystals  in  convergent  light.  In  certain  rock-forming 
minerals  of  this  group  optical  anomalies  are  frequent.  They 
may  be  shown  only  in  convergent  light  by  a  spreading  of  the 
black  cross  upon  rotation,  or  they  may  be  noted  in  parallel  light 
by  the  occurrence  of  double  refracting  segments  in  a  basal  sec- 
tion. Still  these  features  are  not  as  common  as  are  the  anom- 
alies in  isotropic  minerals.  The  circular  polarization  in  quartz 
is  not  observed  in  thin  section. 

In  all  cases  the  shapes  of  the  sections  show  only  the  simplest 
forms  and  this  is  true  to  a  still  higher  degree  for  the  cleavage. 


DESCRIPTIVE  SECTION 


227 


In  the  holohedral  members,  the  cleavage  is  only  parallel  to 
prisms  of  the  first  and  second  order  and  to  the  basal  pinacoid. 
In  hemihedral  forms  rhombohedral  cleavage  in  addition  to  the 
others  is  the  most  important.  The  character  of  the  various 
kinds  of  cleavage  is  shown  schematically  in  Fig.  237. 

Rutile  (4) 

Rutile  is  very  widespread,  but  is  only  present  in  subordinate 
quantities.     It  is  found  in  larger  quantities  only  in  association 
with  certain  intensely  altered  minerals.    It  some- 
times shows  good  crystal  form  which  is  almost 
always  long  prismatic.      Such  development   is 
apparent  even  in  the  minutest  microlites  in  clay 
slates.     It  is  sometimes  found  in  rounded  grains 
or  in  large  compact  masses  that  are  visible  to 
the  naked  eye  and  may  be  recognized  by  the 
blackish-red    color    and    the     submetallic     to 
adamantine    luster.      Twins    parallel  to   (101),       FlG-  238.— Rutile 
Poo   with  knee-shaped  cross  sections,  Figs.  238    doi)poo." 
and  239,  and  those  parallel  to  (301),  3Poo  with 
heart-shaped    cross    sections,  Fig.  240,   are   especially  common 
in  the  smaller  individuals.     In  the  compact  masses  twinning 


Rutile  Twins. 
Parallel  (101)  P  oo  .  Parallel  (301)  3  P  oo  . 


lamination  according  to  the  first  law  is  observed.  Here  also 
prismatic  cleavage,  which  is  often  lacking  in  crystals,  appears 
as  sharp  cracks. 


228  PETROGRAPHIC  METHODS 

Sagenite  is  a  name  given  to  a  characteristic  lattice  structure 
of  rutile  needles  crossing  each  other  at  an  angle  of  about  60°. 
It  is  found  as  an  inclusion  in  numerous  micas,  the  asterism  of 
which  is  often  caused  by  it.  It  appears  in  this  same  form 
produced  by  the  chloritization  of  biotite  rich  in  titanium. 
Rutile  accompanied  by  other  titanium  minerals  is  very  frequently 
produced  by  the  decomposition  of  bisilicates.  Concerning  pleo- 
chroic  halos  around  inclusions  of  rutile  in  biotite,  hornblende, 
cordierite,  etc.,  see  page  197. 

Rutile  has  the  highest  indices  of  refraction  and  double  refraction 
of  all  rock-forming  minerals.  Only  the  smallest  microlites  show 
brilliant  interference  colors,  if  they  do  not  appear  opaque  on 
account  of  total  reflection.  In  some  varieties  the  extraordinary 
ray  is  strongly  absorbed  giving  rise  to  a  change  in  color.  Some- 
times pleochroism  is  entirely  lacking,  especially  in  the  light 
yellow  grains  so  widespread  in  amphibolite  and  eclogite.  The 
larger  compact  particles  are  dark  and  usually  strongly  pleo- 
chroic.  They  show  a  submetallic  luster  in  reflected  light. 

Primary  rutile  is  rare  in  the  eruptive  rocks,  but  is  widely  dis- 
seminated in  all  groups  of  contact  rocks.  In  them  two  types  are 
frequently  associated  with  each  other.  The  one  is  in  sharply 
developed  crystals  with  a  grayish-violet  color,  and  the  other  is 
in  rounded  grains  having  a  yellow  or  brown  color.  It  often  has 
a  border  with  a  crumbly  appearance,  which  is  white  in  reflected 
light  and  consists  predominantly  of  titanite.  This  is  called 
leucoxene.  Rutile  is  sometimes  intergrown  with  homogeneous 
titanite  or  ilmenite  in  various  zonal  arrangements.  It  is  very 
widespread  in  sedimentary  rocks  because  of  its  resistance  to 
weathering.  It  occurs  in  the  coarse  mechanical  sediments  in 
the  form  of  compact  rounded  grains,  and  in  the  fine  sediments  as 
needles  that  are  doubtless  authigenetic.  These  needles  are  very 
minute,  often  twinned  and  scarcely  transparent.  The  most 
typical  development  of  this  formation  appears  to  be  present  only 
in  the  extreme  outer  portion  of  a  contact  zone. 

It  is  riot  attacked  by  the  atmosphere  nor  by  acid,  except  by 
hot  concentrated  sulphuric  acid.  It  can  be  easily  isolated  by  a 
mixture  of  hydrochloric  and  hydrofluoric  acids.  The  test  for 
titanium  is  made  by  fusing  the  residue  in  a  bead  of  potassium 
bisulphate  and  dipping  it  in  hydrogen  peroxide  when  it  becomes 
brownish. 

Very  dark  colored  rutile  might  be  confused  with  opaque  ores. 


DESCRIPTIVE  SECTION  229 

It  can  be  distinguished  by  its  adamantine  luster  in  reflected 
light  and  its  transparency,  that  is  always  to  be  noted  upon  care- 
ful observation.  The  light  colored  varieties  were  formerly  often 
determined  as  zircon.  It  is  distinguished  from  zircon  by  being 
always  colored  and  by  its  higher  double  refraction,  which  also 
differentiates  it  from  anatase.  Distinction  from  the  rarer  min- 
erals brookite,  cassiterite,  and  goethite  is  very  difficult,  as  these 
minerals,  like  rutile,  give  white  of  the  higher  order  in  a  'normal 
section  and  have  an  adamantine  luster  like  it  in  reflected  light. 
The  safest  method  of  differentiation  is  a  chemical  test,  but  the 
effect  on  convergent  light  is  also  distinctive  in  the  case  of  brookite 
and  goethite. 

Rutile  may  be  confused  with  perovskite  and  similar  minerals 
on  account  of  its  high  index  of  refraction,  but  perovskite,  when 
it  is  anomalous,  shows  at  best  a  very  low  interference  color  as 
does  wurtzite,  which  is  also  included  here.  Confusion  with 
sphalerite  may  occur,  but  that  mineral  never  shows  double  re- 
fraction. Titanite  finally  does  not  have  an  adamantine  luster 
in  reflected  light. 

Anatase  (4) 

Anatase  as  a  rock  constituent  is  frequently  developed  tabular,  while 
sharp  pyramidal  forms  are  less  common.  Some  typical  cross  sections  are 
given,  Fig.  237 /,  page  225.  It  may  also  form  granular  aggregates.  The 
individuals  are  mostly  small,  poorly  developed,  clouded,  and  have  a  speckled 
color, — blue,  yellow  or  colorless.  The  stronger  absorption  of  the  ordinary 
ray  is  not  often  clearly  discernable,  and  the  same  is  true  of  the  cleavage.  It 
generally  results  from  the  alteration  of  other  minerals  containing  titanium 
and  is,  therefore,  found  particularly  in  greatly  altered  rocks,  in  contact- 
metamorphic  formations,  as  well  as  in  those  which  have  suffered  decomposi- 
tion by  the  action  of  mineralizers.  It  is  very  common  in  such  rocks  as  a 
by-product  in  the  alteration  of  bisilicates  and  it  forms  a  part  of  the  dense 
aggregate  known  as  leucoxene.  It  is  confused  with  zircon,  from  which  it 
can  be  distinguished,  however,  by  the  speckled  color  and  cloudy  appearance, 
much  higher  indices  of  refraction,  and  the  optical  character.  It  is  also 
similar  to  rutile,  and  titanite  from  which  it  is  distinguished  by  its  much 
lower  double  refraction,  imparting  to  it  brilliant  interference  colors  in  the 
section. 

It  may  be  remarked,  parenthetically,  that  grains  of  corundum  similar 
to  anatase  may  be  present  in  the  section.  These  have  been  derived  from  the 
polishing  or  grinding  material.  The  optical  properties  of  these  grains  are 
similar  to  those  of  anatase.  They  are  uniaxial,  blue  or  yellowish,  and 
pleochroic,  but  differ  in  the  optical  character  of  the  principal  zone. 


230  PETROGRAPHIC  METHODS 

Cassiterite     (4) 

Cassiterite  is  a  rare  constituent  of  lithionite  granite  and  the  rocks  in  the 
vicinity  of  a  contact  with  it.  Crystals  are  not  frequent  in  these  rocks,  but 
they  have  pyramidal  development  with  characteristic  twinning  parallel  to 
(101),  P  00  or  they  may  be  simple  prisms.  Grains  are  the  most  common.  It 
is  rarely  colorless,  but  is  yellow,  brown,  or  more  often  deep  red.  In  the 
last  case  it  is  strongly  pleochroic  in  thin  section.  A  zonal  or  speckled  dis- 
tribution of  the  color  is  common.  If  its  determination  is  doubtful,  especially 
if  it  is  confused  with  rutile  and  anatase,  the  grains  should  be  isolated  with 
hydrofluoric  and  sulphuric  acids  and  fused  in  a  borax  bead  colored  blue  by 
copper  oxide.  A  ruby-red  color  then  indicates  cassiterite.  (See  page  170.) 

Wurtzite  (4) 

Wurtzite  is  found  together  with  sphalerite  and  is  intergrown  with  it  in 
clustered  incrustations.  It  is  similar  to  sphalerite  in  color.  Its  distri- 
bution as  a  rock-forming  mineral  has  not  been  definitely  determined.  It  is 
distinguished  from  the  minerals,  which  appear  similar  to  it,  rutile,  goethite, 
and  sphalerite,  by  its  weak  double  refraction.  Its  solubility  in  acids  is  also 
distinctive. 

Zircon(4) 

Zircon  is  one  of  the  most  widespread  rock  constituents,  but 
is  always  present  only  in  small  quantities.  It  is  macroscopically 
apparent  only  in  pegmatites.  The  so-called  zircon  syenites, 
belonging  to  the  soda  series  of  rocks,  are  especially  rich  in  it. 
The  mineral  is  abundantly  present  in  brown,  red,  or  greenish 
cloudy  crystals  with  an  adamantine  luster  and  forms  an  import- 
ant constituent  of  the  rock.  As  a  rock  constituent,  it  is  always 
microscopic  and  the  individuals  are  in  most  cases  very  small.  It 
is  entirely  colorless  in  thin  section.  It  is  almost  never  wanting 
in  the  acid  and  intermediate  members  of  the  normal  series  of 
eruptive  rocks  and  is  one  of  the  first  minerals  to  crystallize  in 
them.  It  is,  therefore,  in  minute  crystals  that  are  as  perfect 
as  models  and  often  have  an  abundance  of  faces.  It  is  less  com- 
mon in  basic  eruptive  rocks  and  is  only  exceptionally  observed 
in  peridotites  and  in  the  members  of  the  soda  rock  series  rich  in 
alkali.  It  is  found  in  all  types  of  contact  rocks,  but  it  occurs  more 
frequently  in  grains,  which  are  well  characterized  by  their  high 
index  of  refraction  and  brilliant  interference  colors.  It  is  nearly 
always  present  in  sedimentary  rocks,  but  mostly  in  clouded  grains. 

Zircon  is  not  attacked  by  acids.  When  decomposed  in  rocks 
it  often  shows  characteristic  zonal  structure  in  which  clear  and 
more  or  less  clouded  zones  alternate,  Fig.  241.  In  spite  of  this 


DESCRIPTIVE  SECTION 


231 


there  is  much  doubt  concerning  the  miner alogical  and  petro- 
graphical  definition  of  the  mineral.  It  is  often  very  difficult  to 
distinguish  it  from  xenotime,  which  has  a  slightly  higher  double 
refraction  and  according  to  modern  investigations  is  found  to  be 
as  widespread  in  acid  rocks  as  zircon.  Because  of  their  similar 
appearance  and  occurrence  they  can  be  differentiated  positively 
only  by  the  Hepar  reaction.  Besides  that,  very  different  varie- 
ties can  be  distinguished  in  macroscopic  crystals  of  zircon.  One 
variety  with  a  normal  specific  gravity  and  quite  a  high  double 
refraction  seems  to  be  related  by  gradual  transi- 
tions with  one  whose  specific  gravity  sinks  to  4, 
and  which  is  scarcely  double  refracting  "when 
entirely  clear  and  homogeneous.  It  cannot  be 
determined  just  how  much  this  transition  is 
based  upon  the  taking  up  of  water,  as  has  been 
shown  for  the  cloudy  variety  known  as  malacon. 
Little  is  known  of  the  distribution  of  the  varie- 
ties with  low  double  refraction  because  the 
mineral  occurs  in  small  amounts  and  is  usually 
overlooked,  or  is  classed  as  a  member  of  the 
epidote  series. 

Xenotime  is  frequently  found  by  chemical 
investigations  among  the  high  double  refracting 
individuals  and  is  generally  determined  as 
zircon.  Sometimes  the  zonal  clouded  crystals 
belong  principally  to  xenotime.  Under  certain  conditions  it 
can  be  confused  with  colorless  cassiterite,  but  the  chemical 
reaction  and  higher  double  refraction  of  cassiterite,  giving 
rise  to  white  of  the  higher  order  in  a  normal  section,  serve  to 
distinguish  them.  It  is  distinguished  from  titanite  by  a  lower 
double  refraction.  It  can  also  be  confused  with  monazite  or 
with  colorless  epidote,  but  convergent  light  shows  the  difference 
at  once.  Confusion  with  anatase  and  rutile  was  mentioned 
under  these  minerals.  The  occurrence  of  pleochroic  halos 
around  zircon  individuals  is  noteworthy,  see  page  197.  Very 
perfect  cleavage  is  only  clearly  observed  in  the  larger  individuals 
and  is  almost  entirely  lacking  in  the  ordinary  microlitic  forms. 

Xenotime  (4) 

The  distribution  of  xenotime  in  rocks  cannot  be  ascertained  with  our 
present  knowledge  of  it.     It  appears  to  be  present  first  of  all  in  acid  eruptive 


FIG.  241. — Zircon 
(Xenotime).  Sec- 
tion Parallel  to  the 
Principal  Axis. 


232 


PETROGRAPHIC  METHODS 


rocks  and  in  sedimentary  rocks  derived  from  them,  occurring  with  or  in- 
stead of  zircon.  It  is  easily  recognized  when  it  occurs  in  short  pyramidal 
crystals,  which  are  sometimes  intergrown  parallel  with  prismatic  zircon 
crystals.  It  may  form  prismatic  crystals  itself  like  Fig.  241  and  these  can 
scarcely  be  distinguished  from  zircon.  The  higher  interference  colors, 
white  of  the  higher  order  if  the  crystals  are  not  too  thin,  and  also  the  pleo- 
chroism  (o>  light  rose-red,  e  pale  yellowish),  which  closer  observation  reveals, 
make  the  recognition  of  isolated  crystals  possible,  but  in  thin  section  both 
of  these  properties  are  not  distinctly  evident.  The  pyramidal  crystals  are 
usually  greatly  clouded  by  decomposition  and  in  the  prismatic  crystals, 
which  were  simply  called  zircon,  this  turbidity  is  very  common,  particularly 
in  the  zonal  development.  Clear  prismatic  crystals  are  rarer.  The  Hepar 
test  on  carefully  isolated  material  gives  positive  proof  of  xenotime.  How- 
ever, only  fresh  varieties  give  this  reaction  because  the  sulphuric  acid  disap- 
pears when  the  crystals  become  cloudy.  It  is  difficultly  fusible  before  the 
blowpipe,  and  if  moistened  with  sulphuric  acid  it  colors  the  flame  bluish- 
green.  It  is  slowly  decomposed  in  a  salt  of  phosphorous  bead. 


Corundum  (4) 

Corundum  shows  varying  habits.  Short  pyramidal  crystals  sometimes 
with  a  barrel  shape,  Fig.  242,  and  with  distinct  zonal  structure,  or  thin 
tabular  crystals  parallel  to  the  base  are  the  common  forms.  Cross  sections 
with  a  poor  outline  and  pale  speckled  color  are  often 
difficult  to  determine,  because  they  have  no  character- 
istic appearances  except  the  high  index  of  refraction. 
The  pleochroism  is  characteristic  and  is  observed  only 
when  the  mineral  has  a  deep  blue  color.  The  various 
other  macroscopic  colors  are  not  seen  in  a  thin  section. 
A  few  twin  lamellae  parallel  to  the  rhomb ohedron  are 
seen  now  and  then.  As  a  granular  aggregate  it  forms 
the  principal  constituent  of  emery,  which  presumably 
represents  a  dike-like  development  in  contact-metamor- 
phic  limestones.  It  is  otherwise  almost  entirely  a  con- 
tact mineral  and  as  such  is  most  frequently  developed 
tabular  and  accompanied  by  sillimanite,  cordierite, 
spinel,  etc.;  or  it  occurs  as  the  result  of  resorbed  inclu- 
sions of  aluminous  rocks  in  eruptive  rocks.  These  are 
the  large  blue  crystals  of  sapphire  occurring  especially 
in  pegmatites.  It  is  also  found  in  contact  rocks  as 
dark  irregular  masses  of  microscopic  size  produced  by 
segregation.  Corundum  can  be  recognized  by  the  naked  eye  in  granular 
limestones  and  dolomites  in  which  it  appears  brilliant  red,  ruby,  but  in 
thin  section  it  is  colorless.  It  is  often  necessary  to  isolate  corundum 
from  a  rock  to  determine  it  positively.  Its  great  resistance  even 
toward  molten  carbonates  of  the  alkalies  makes  its  isolation  easy.  The 
presence  of  small  grains  of  corundum  in  sections  prepared  with  emery  is 
mentioned  because  they  often  give  rise  to  confusion.  Corundum  is  dis- 
tinguished from  vesuvianite  and  apatite  by  its  more  brilliant  interference 


FIG.  242.— Barrel- 
shaped  Crystal  of 
Corundum. 


DESCRIPTIVE  SECTION  233 

colors.  Light  colored  varieties  of  tourmaline  can  be  very  similar  to  it,  but 
the  latter  shows  a  greater  difference  in  absorption.  Among  the  biaxial 
minerals  it  may  be  confused  with  those  which  are  often  blue  like  corundum, 
such  as  cyanite,  sapphirine,  lawsonite,  zoisite,  sillimanite,  etc.  Investiga- 
tions in  convergent  light  show  the  differences. 

Vesuvianite  (lodocrase)  (4) 

Vesuvianite  is  a  typical  contact  mineral  that  occurs  in  well  developed 
short  prismatic  crystals  only  in  contact-metamorphosed  limestones.  In 
other  contact  rocks  it  forms  irregular,  elongated  grains.  It  is  macroscopic- 
ally  light  green  or  yellow  to  brown  in  color,  but  in  thin  section  it  is  colorless 
and  transparent.  Only  those  varieties  containing  manganese  are  colored 
distinctly  reddish.  It  is  decomposed  by  hydrochloric  acid  with  difficulty 
and  fuses  before  the  blowpipe  with  intumescence  to  a  greenish  glass.  After 
fusion  it  gelatinizes  with  hydrochloric  acid.  The  vesuvianites  form  a  series 
of  isomorphous  mixtures  of  which  one  end  member,  which  usually  predomi- 
nates, is  optically  negative,  and  the  other  has  a  weak,  positive,  double  re- 
fraction. Anomalous  interference  colors  are  therefore  frequently  observed 
in  the  intermediate  members  and  the  colors  may  be  arranged  in  a  zonal 
manner.  The  division  of  basal  sections  into  regular  biaxial  fields  is  very 
common.  Dense  aggregates  like  nephrite,  which  occur  in  serpentine  and 
produce  a  formation  similar  to  saussurite,  are  noteworthy.  In  such  an 
occurrence  it  is  extremely  difficult  to  recognize  Vesuvianite  and  distinguish 
it  from  grossularite,  gehlenite  or  zoisite,  and  clinozoisite,  since  all  these 
minerals  have  high  indices  of  refraction  and  show  anomalous  interference 
colors.  It  is  difficult  to  distinguish  from  apatite  and  this  can  often  only  be 
done  by  a  chemical  test.  It  appears  similar  to  andalusite  even  when  it  has 
normal  interference  colors,  but  the  biaxial  behavior  of  the  latter  serves  to 
differentiate  the  two. 

Gehlenite  Group  (4) 

Gehlenite  and  melilite  are  apparently  an  isomorphous  mixture  of  a  cal- 
cium-aluminium silicate,  which  sometimes  occurs  pure  in  gehlenite,  with 
a  calcium  silicate  free  from  aluminium,  that  is  not  known  in  nature.  Gehle- 
nite forms  rounded,  short,  prismatic  crystals  in  contact-metamorphic  lime- 
stones, and  the  length  is  usually  quite  the  same  as  the  thickness.  Melilite 
occurs  more  frequently  in  poorly  bounded,  tabular  crystals  parallel  to  the 
base,  and  appears  to  be  confined  to  basalts  and  tephrites.  The  former  is 
macroscopically  grayish,  and  in  thin  section  always  colorless.  The  latter  is 
now  and  then  colored  yellow  by  its  content  of  iron  and  the  extraordinary 
ray  is  absorbed  more  than  the  other — humboldite.  The  latter  also  occurs  in 
large  irregular  particles  which  are  entirely  perforated  by  leucite  crystals. 
The  negative  double  refraction  diminishes  with  a  decrease  in  the  content 
of  aluminium  and  those  poorest  in  aluminium  are  optically  positive.  Be- 
tween these  extremes  there  is  a  series  of  mixtures,  which  show  anomalous 
interference  colors  in  the  most  typical  manner. 

Both  minerals  are  easily  altered  and  the  melilite  particularly  is  often 


234 


PETROGRAPHIC  METHODS 


coated  with  a  dull  hazy  film.  The  mineral  itself  is  also  clouded  considerably. 
It  may  pass  over  into  a  fibrous  aggregate  with  a  strong  double  refraction, 
the  fibers  standing  perpendicular  to  the  basal  pinacoid.  Melilite  often 
shows  the  characteristic  appearance  of  having  its  sections  crossed  by 
numerous  thin  glass  pegs  arranged  parallel  to  the  principal  axis.  These 
penetrate  into  the  crystals  from  both  sides  and  often  spread  out  like  funnels 
in  the  interior.  Such  an  appearance  is  called  peg  structure,  Fig.  243.  Cleav- 
age is  often  entirely  lacking  in  such 
sections,  but  in  other  cases  it  is 
seen  in  a  few  sharp  cracks  running 
perpendicular  to  the  principal  axis. 
Perovskite  is  a  characteristic  asso- 
ciate of  melilite.  Gehlenite  is  often 
very  similar  to  vesuvianite  even  in 
varieties  with  anomalous  interfer- 
ence colors— fuggerite — and  to 
zoisite  and  clinozoisite,  and  is  often 
only  to  be  distinguished  from  them 
positively  by  treatment  with  hydro- 
chloric acid.  Melilite  was  formerly 
determined  as  feldspar  or  as  neph- 
eline,  but  is  distinguished  from 
them  by  the  higher  index  of  refrac- 
tion, the  dull  appearance  in  re- 
flected light,  and  the  chemical 
properties,  particularly  the  ease 
with  which  it  is  dissolved  in  acids,  and  the  high  content  of  calcium. 

In  certain  contact  rocks  in  the  Fassatal,  gehlenite  is  replaced  by  fuggerite, 
a  mineral  with  a  similar  composition.  It  is  likewise  tetragonal  and  occurs 
in  tabular  crystals.  It  is  isotropic  for  sodium  light  and  therefore  has  deep 
blue  anomalous  interference  colors.  It  has  a  more  perfect  basal  cleavage. 
Sp.  gr.  =  3.18.  It  is  soluble  in  acids  with  the  separation  of  powdered  silica. 

Tourmaline  (4) 

A  group  of  complex  silicates  containing  boric  acid  is  included 
under  the  name  of  tourmaline.  Lithia  tourmaline,  macro- 
scopically  very  pale,  in  thin  section  colorless,  scarcely  ever 
occurs  as  a  real  rock  constituent  and  is  distinguished  from  the 
strongly  colored  green,  blue  or  brown  magnesia  tourmalines, 
which  are  mostly  light  colored  in  thin  section.  The  iron  tourma- 
line containing  titanium,  schorl,  is,  like  most  silicates  containing 
titanium,  macroscopically  black  with  a  pitchy  luster.  It  is 
highly  colored  in  thin  section  and  is  characterized  by  a  very 
strong  absorption  of  the  ordinary  ray.  It  is  the  most  important 
member  of  the  series  as  a  rock  constituent. 

Schorl  is  often  in  zonal  crystals  built  up  of  layers  with  different 


FIG.  243.— Peg  Structure  in  Melilite. 
Melilitebasalt,  Oahu,  Sandwich  Islands. 
(After  E.  Cohen.) 


DESCRIPTIVE  SECTION 


235 


intensities  of  color.  The  hemimorphic  development  of  the 
crystals,  the  ends  of  which  are  shown  diagrammatically  in  Figs. 
244  and  245,  is  rarely  apparent,  but  the  three-  to  nine-sided 
cross  sections,  Fig.  246,  are  very  characteristic.  The  tourma- 
lines that  are  light  colored  in  thin  section  are  especially  wide- 
spread in  granular  limestones  and  are  often  found  in  large, 
well  developed  crystals  with  yellowish,  greenish,  bluish  or 
brownish  color,  but  these  are  often  difficult  to  recognize  because 
of  the  comparatively  small  difference  in  absorption.  In  other 
rocks  the  blue  varieties  form  ragged  particles  without  any  indi- 


FIG.  244. 


FIG.  245. 


Opposite  Ends  of  a  Tourmaline  Crystal. 


FIG.  246. — Tourmaline 
Cross  Section. 


cation  of  crystal  form.  Radial  aggregates — tourmaline  suns — 
are  very  widespread.  It  is  often  noted  also  that  a  crystal  of  dark 
tourmaline  is  well  bounded  on  one  side  and  on  the  other  side  it 
is  grown  into  a  fibrous  aggregate  of  light  colored  tourmaline. 

The  optical  properties  vary  within  wide  limits.  The  indices  of 
refraction  and  the  double  refraction  appear  to  be  highest  in  the 
members  rich  in  titanium.  The  indices  and  the  double  refraction 
increase  in  the  pleochroic  halos,  see  page  197. 

The  dark  colored  varieties  are  sufficiently  distinguished. in  all 
cases  from  the  numerous  and  widespread  members  of  the  mica 
and  amphibole  groups  and  from  apatite,  with  which  they  have 
been  confused,  by  the  strong  absorption  perpendicular  to  the 
principal  zone  and  by  the  trigonal  cross  sections.  The  minerals 
with  light  color  and  low  absorption,  which  are  difficultly  recog- 
nizable, are  very  similar  to  andalusite,  staurolite,  lawsonite, 
forsterite,  and  corundum,  but  by  more  careful  observation 
the  stronger  absorption  of  the  ordinary  ray  can  be  observed 
even  though  it  is  only  seen  in  traces.  Epidote  often  shows  a 
similar  orientation  of  the  absorption.  In  this  case  the  usual 
speckled  appearance  of  the  interference  colors  of  epidote  and  its 
cleavage  serve  to  differentiate  them.  Investigation  in  conver- 
gent polarized  light  is  helpful  here  as  in  most  of  the  other  cases. 


236  PETROGRAPHIC  METHODS 

In  any  case  the  resistance  of  tourmaline  to  reagents  allows  it  to 
be  easily  isolated  and  it  can  be  determined  by  its  reaction  for 
boron,  see  page  169. 

Only  schorl  is  known  as  a  primary  constituent  of  eruptive  rocks 
and  it  sometimes  occurs  as  an  accessory  constituent  of  granitic 
rocks,  especially  the  aplites.  Otherwise  tourmaline  is  the  most 
distinctive  mineral  of  pneumatolytic  processes,  and  its  formation 
can  nearly  everywhere  be  shown  to  be  connected  with  such 
processes.  It  is  therefore  always  found  where  they  have  been 
most  intensely  active.  Besides  in  the  pegmatites,  it  is  found 
especially  in  the  vicinity  of  tin  ore  veins  and  certain  copper  ore 
dikes,  where  the  whole  rock  has  been  tourmalinized.  It  occurs 
also  in  the  kaolin  deposits. 

The  distribution  of  tourmaline  in  contact  rocks  of  all  sorts 
must  be  especially  emphasized.  It  is  never  lacking  in  all  the 
various  kinds  of  formations  which  owe  their  origin  to  contact- 
metamorphic  alteration  by  granitic  rocks,  even  though  the 
granite  itself  contains  no  trace  of  tourmaline.  Tourmaline 
sometimes  occurs  in  very  large  individuals,  e.g.,  in  the  schist 
zone  of  the  Central  Alps  where  the  crystals  are  macroscopically 
apparent,  but  even  in  these  rocks  it  is  more  common  in  separate, 
minute  microlites,  which  could  be  easily  overlooked  unless  one 
accidentally  observes  a  typical  cross  section  or  finds  a  pris- 
matic crystal  with  the  characteristic  absorption  perpendicular  to 
the  principal  zone.  Tourmaline  is  everywhere  present  in  this 
form  in  gneisses,  mica  schists,  amphibolites,  green  schists, 
hornfels,  and  Knotenschiefer  and  it  is  found  in  distinctly 
developed  individuals  in  the  remotest  parts  of  the  contact  zone. 
Here  no  alteration  can  be  recognized  in  the  external  appearance 
of  the  rock  itself,  and  under  the  microscope  the  action  of  contact 
metamorphism  can  only  be  discerned  by  the  formation  of  tour- 
maline and  the  development  of  small,  clay  slate  needles — rutile. 

Apatite  (5) 

Apatite  is  everywhere  present  as  a  rock  constituent,  but  in 
small  quantities  and  nearly  always  in  minute  individuals.  It 
occurs  in  acid  eruptive  rocks  in  well  developed  crystals  with  a 
long,  prismatic  to  needle-like  habit,  Fig.  247.  In  basic  rocks 
and,  especially  in  those  rich  in  sodium,  it  forms  large,  rounded 
crystals  that  are  short  and  thick,  and  in  contact  rocks  and  sedi- 


DESCRIPTIVE  SECTION 


237 


FIG.  247. 
Apatite. 


ments  it  forms  rounded  grains.  Apatite  as  a  rock  constituent 
can  only  be  seen  macroscopically  in  exceptional  cases.  It  appears 
as  crystals  with  a  brilliant  luster  in  certain  camptonites  and  may 
likewise  be  observed  as  light  blue  grains  in  marble.  Radial 
aggregates — phosphorite — frequently  colored  dark  by 
organic  substances,  are  found  in  numerous  sediments 
as  large  concretions. 

Apatite  is  always  one  of  the  first  products  to  crys- 
tallize in  eruptive  rocks  and  is,  therefore,  largely  in- 
cluded in  the  other  constituents,  especially  the  dark 
ones.  It  is  strikingly  contrasted  with  the  other 
minerals  by  its  colorless  lath-shaped  or  six-sided  cross 
sections,  Fig.  216,  page  197.  Gas,  liquid,  and  glass 
are  observed  as  inclusions  in  apatite.  It  is  not  often 
colored  and  the  colored  varieties  are  confined  to 
eruptive  rocks.  It  may  be  grayish-blue,  brown,  or 
brilliant  orange,  with  a  strong  absorption  parallel  to  the  principal 
zone.  Six-sided  sections  do  not  give  a  distinct  interference  figure 
because  of  the  low  double  refraction.  Weathering  and  disin- 
tegration of  the  rock  usually  leave  the  apatite  unaffected.  It  is 

therefore  found  very  wide- 
spread in  soils,  which  owe  their 
content  of  phosphate  to  the 
great  dissemination  of  apatite 
microlites. 

In  many  instances  apatite 
can  only  be  distinguished  posi- 
tively from  vesuvianite,  zois- 
ite,  or  from  other  colorless 
double  refracting  grains  be- 
longing to  orthite  (allanite)  by 
chemical  tests  for  phosphoric 
acid.  Low  double  refraction 
and  the  negative  character  of 
the  principal  .zone  distinguish  it 
from  tremolite,  sillimanite,  etc. 


FIG.  248. — Slide  of  a  Bone  with  Perfectly 
Retained  Structure  from  the  Permian  Wichita 
Beds  of  Texas. 


Bone  substance,  consisting  predominantly  of  calcium  phosphate,  is  to  be 
appended  to  apatite.  It  likewise  has  a  low  double  refraction  and  occurs  in 
fine  fibrous  aggregates  that  are  imperfectly  radial.  Within  them  the  or- 
ganic structure  is  often  perfectly  retained  and  shows  itself  in  the  nutrition 
canals,  etc.,  Fig.  248. 


238 


PETROGRAPHIC  METHODS 


Rhombohedral  Carbonates  (5) 

The  rhombohedral  modifications  are  the  only  carbonates  that 
occur  as  actual  rock  constituents.  The  others  are  found  prin- 
cipally as  secondary  depositions  in  crevices  and  as  organic  remains. 
The  diagenetic  transformation  of  these  into  calcite  is  explained 
in  the  Allgemeine  Gesteinskunde  by  E.  Weinschenk,  page  118. 
Calcite  is  one  of  the  most  widespread  rock-forming  minerals.  It 
forms  the  principal  constituent  of  extensive  formations  and  con- 
stitutes whole  mountain  chains.  Dolomite  is  likewise  quite 
widespread,  while  the  other  carbonates  treated  here,  magnesite, 
siderite,  and  smithsonite,  are  more  of  local  importance. 

It  is  noteworthy  that  calcite  seldom  forms  crystals  in  rocks. 
They  do  occur  in  minute,  isolated  rhombohedrons  in  the  central 
granite  and  are  here  undoubtedly  grown  into  the  quartz  as  a 
primary  constituent.  Other  than  this  it  is  always  a  secondary 
constituent  of  eruptive  rocks  produced  by  the  alteration  of  sili- 
cates of  lime.  Pseudomorphs  of  calcite  after  plagioclase  or 
augite,  and  even  after  olivine,  are  found  in  greatly  altered  occur- 
rences. It  is  especially  frequent  under  such  conditions  as  an 

impregnation  in  the  rocks, 
which  it  may  penetrate  in 
large  veins  in  which  the  calcite 
is  granular,  or  fibrous  or  the 
ground  mass  of  a  porphyric 
rock  may  be  entirely  impreg- 
nated with  fine  aggregates  of 
calcite,  which  frequently  pos- 
sess a  radial,  fibrous  structure. 
Dense  sedimentary  lime- 
stones consist  predominantly 
of  irregular,  granular  aggre- 
gates of  calcite.  The  indi- 
viduals are  usually  clouded 
with  inclusions  and  vary 
greatly  in  size.  They  are  often  transformed  into  larger  grains 
with  greater  clearness  by  recrystallization.  The  organic  struc- 
ture is  often  distinctly  retained  in  such  a  formation,  Fig.  249. 
Only  rarely  do  the  limestones  fail  to  show  crystalline  aggregates 
under  the  microscope,  as  for  example  in  the  Solenhofener  schists, 
which  are  considered  as  a  brackish  water  formation.  Radial, 


FIG.    249. — Crinoid  Limestone  with   Lattice 
Structure.     Vilstal  near  Pfronten,  Algan. 


DESCRIPTIVE  SECTION  239 

fibrous  oolites,  sometimes  with  onion  structure,  are  found  in 
limestones.  They  often  lose  their  structure  by  secondary 
processes  and  then  they  consist  of  small  spheres  of  granular 
calcite,  Figs.  190  and  191,  pages  186-187.' 

Calcite  shows  its  best  development  in  the  various  groups  of 
contact  rocks,  as  in  lime-mica  schist  and  in  calcium-silicate  fels. 
In  some  of  these  rocks  it  forms  the  principal  constituent  as  in 
marble,  while  in  others  it  occurs  in  subordinate  particles  as  in  the 
two  rocks  mentioned  above.  It  never  shows  crystal  form,  but  is 
always  in  granular  aggregates  and  all  the  other  rock  constit- 
uents are  well  crystallized  in  it,  while  in  those  occurrences  rich 
in  silicates  the  calcite  takes  on  the  character  of  interstitial  ma- 
terial. Pure  marble  lying  next  to  the  contact  is  often  quite 
coarse  grained,  the  individuals  measuring  as  much  as  an  inch  in 
one  dimension  and  these  tend  to  be  colored  sky  blue.  All  gra- 
dations are  found  from  this  down  to  formations  that  are  macro- 
scopically  entirely  non-crystalline.  Under  the  microscope  a  very 
regular  texture  of  crystalline  grains  of  calcite  is  observed.  It 
sometimes  shows  mosaic  structure  and  sometimes  the  grains 
are  intimately  dovetailed  into  each  other,  Figs.  192  and  193,  page 
187.  The  individual  grains  are  sometimes  clouded  with  fine 
graphite  dust  and  then  are  somewhat  pleochroic  with  the  darker 
color  in  the  vibration  direction  of  the 
ordinary  ray.  The  perfect  cleavage 
parallel  to  the  unit  rhombohedron,  Fig. 
250,  which  is  difficult  to  observe  in  the 
denser  aggregates,  shows  plainly  in  this 
type  in  a  series  of  sharp  cracks.  Very 
frequently,  but  not  always,  numerous 
twinning  lamellae  parallel  to  —  £R  are  FlG.  250.— Rhombohedron. 
seen,  and  these  are  often  parallel  to  but 

one  face  of  that  form.  They  are  to  be  explained  by  the  gliding 
of  the  calcite  under  the  influence  of  pressure,  and  have  proba- 
bly resulted  during  the  mechanical  operations  of  the  grinding. 
The  small  number  of  lamellae  in  Figs.  192  and  193  is  remarka- 
ble, and  is  very  distinctive  because  the  marble  from  Carrara  is 
considered  as  a  type  of  dynamo-metamorphic  limestone. 

Undoubtedly  the  twinning  lamellae  are  increased  everywhere 
where  the  marble  has  been  subjected  to  oro genie  stresses  after  re- 
crystallization  had  taken  place,  and  such  occurrences  consist  .en- 
tirely of  fibrous  grains  in  which  the  manifold  bending  indicates 


240  PETROGRAPHIC  METHODS 

the  high  degree  of  plasticity  of  calcite,  Fig.  205,  page  192.  This 
mineral  also  breaks  under  too  high  a  pressure  and  dense  shattered 
aggregates  with  great  solidity  are  developed  from  granular  marble. 
This  is  well  illustrated  in  the  ivory  marble  with  its  excellent 
cataclastic  appearance. 

The  great  difference  in  the  indices  of  refraction  for  the  two 
principal  vibration  directions  can  be  distinctly  shown  under 
the  microscope  with  one  nicol.  It  is  also  the  cause  of  the  brilliant 
interference  colors,  which  the  twinning  lamella  crossing  the  sec- 
tion obliquely  often  show;  compare  Part  I,  page  87.  In  a 
normal  section  calcite  gives  a  pale  white  of  the  higher  order 
between  crossed  nicols.  Cross  sections  in  which  the  cleavage 
cracks  form  an  equilateral  triangle  show  an  interference  figure 
distinctly  even  with  a  low-power  objective.  With  a  higher 
objective  a  black  cross  is  obtained  surrounded  by  numerous 
colored  rings  but  these  are  often  disturbed  by  interbedded 
twinning  lamellae. 

Dolomite  is  distinguished  from  calcite  by  its  greater  tendency 
to  develop  its  own  crystal  form.  If  it  is  intergrown  in  calcite 
it  usually  shows  rhombohedral  cross  sections.  Aggregates  con- 
sisting principally  of  dolomite  show  the  mosaic  structure  much 
more  distinctly  than  calcite,  and  this  may  pass  over  into  a  fine 
drusy  granular  structure  like  sugar.  The  dovetailed  structure 
is  also  found  in  pure  dolomites  but  it  is  rare.  It  forms  worm- 
like  intergrowths  in  calcite  in  a  few  occurrences  of  eozoon  cana- 
densis.  The  higher  indices  of  refraction  and  double  refraction 
compared  with  calcite  cannot  generally  be  determined  in  an 
ordinary  section.  The  lack  of  twinning  lamellae  parallel  to  —  1/2R 
is  typical,  for  that  is  not  a  gliding  plane  in  dolomite.  In  place 
of  that,  however,  twinning  parallel  to  —  2R  occurs  in  numerous 
lamellae  so  that  is  not  a  good  means  of  differentiation  although 
generally  there  are  fewer  twinning  lamellae  in  dolomite  than  in 
calcite.  Furthermore,  they  are  seldom  so  greatly  deformed. 
The  greater  absorption  of  the  ordinary  ray  is  more  easily  noted 
than  in  calcite. 

Dolomite  is  also  found  .as  a  secondary  formation  in  altered 
eruptive  rocks  and  may  be  in  pseudbmorphs  or  as  fine  impreg- 
nations in  the  ground  mass.  If  the  content  of  iron  which  is 
always  present  has  not  given  rise  to  the  formation  of  rust, 
and  also  in  the  rare  cases  where  it  occurs  in  radial  fibrous  form 
filling  out  crevices,  dolomite  can  only  be  determined  by  a  chem- 


DESCRIPTIVE  SECTION  241 

ical  test.  Dolomite  rocks  of  the  sedimentary  formations  gener- 
ally show  a  more  distinct  crystalline  texture  than  limestones, 
and  are  often  penetrated  through  and  through  by  veins  of  calcite. 
The  numerous  cavaties,  which  appear  macroscopically  and  are 
covered  with  small  dolomite  crystals,  are  especially  character- 
istic. Under  the  influence  of  contact  metamorphism  the  dolo- 
mites form  granular,  and  often  pure,  white  dolomite  marble, 
which  is  usually  finer  grained  than  the  equivalent  limestones, 
and  like  the  limestones  show  transitions  into  silicate  fels.  Gran- 
ular dolomite  with  excellent  mosaic  structure  weathers  very 
frequently  to  a  sandy  mass  consisting  of  small  rhombohedrons, 
dolomite  ash. 

Magnesite  is  much  rarer  and  only  of  local  importance.  If  it  results  as  a 
by-product  of  serpentinization  it  is  found  in  well  developed  unit  rhombo- 
hedrons containing  considerable  iron.  They  are  intergrown  in  serpentine, 
chlorite  fels,  pot  stone,  etc.  It  forms  granular  pseudomorphs  after  olivine 
in  certain  melaphyres  and  in  sagvandite.  Larger  homogeneous  masses, 
which  occur  within  the  metamorphic  limestones  in  the  border  zones  of  the 
Central  Alps  have  a  very  coarse-grained  texture  and  consist  of  flat  rhombo- 
hedrons, —  1/2R,  with  dimensions  of  about  an  inch.  This  form  occurs  very 
distinctly  in  pinolites  streaked  with  layers  of  clay  slate.  These  varieties 
also  contain  iron  in  considerable  amounts  and  they,  therefore,  assume  a 
rusty  appearance  upon  weathering.  Snow-white  aggregates  of  magnesite, 
free  from  iron,  are  just  the  opposite  of  those  above.  They  are  dense  like 
porcelain,  have  a  conchoidal  fracture,  and  form  veins  in  serpentine  and,  if 
it  can  be  detected  microscopically  at  all,  they  show  a  spherulitic  texture. 

Siderite  is  sometimes  yellowish  and  weakly  pleochroic  in  thin  sections.  It 
occurs  locally  in  extensive  deposits,  and  in  geological  association  with 
magnesite  it  forms  coarse  to  medium  grained  masses  within  the  granular 
limestones  of  the  Central  Alps.  It  is  also  found  as  a  constituent  of  carbona- 
ceous iron  rock  occurring  in  bedded  deposits.  It  is  fine  granular  and  filled 
with  carbonaceous  particles.  It  occurs  also  in  the  form  of  fibrous  and  spher- 
ulitic concretions — spherosiderite.  This  name  is  also  used  for  radial  fibrous 
incrustations  of  iron  carbonate  in  crevices  of  trap  rock. 

For  the  other  modifications  of  calcium  carbonate,  see  aragonite  under  the 
biaxial  minerals.  Ktypeite  has  been  found  in  rocks,  especially  as  a  constit- 
uent of  a  few  pisolites.  It  has  a  specific  gravity  of  about  2. 65  and  n  about 
1,55.  f—  a  =  0.020,  optically  uniaxial,  positive.  Conchite,  which  is  quite 
similar  to  aragonite  in  many  respects  but  is  undoubtedly  not  the  same,  is  of 
interest  because  it  forms  the  principal  constituent  of  shells  of  living  mollusks 
and  many  other  lime  organisms.  Its  geological  significance  depends  upon 
its  slight  stability,  especially  upon  the  ease  with  which  it  is  transformed  on 
the  one  hand  into  calcite  and  on  the  other  into  dolomite  under  the  influence 
of  solutions  containing  magnesium.  Sp.  gr.  =2.85,  a  =  1.523,  ^  =  1.662, 
f—  a  =  0.139.  Optically  uniaxial,  negative,  but  frequently  shows  a  distinct 
opening  of  the  cross  in  convergent  polarized  light. 
16 


242  PETROGRAPHIC  METHODS 

Concerning  the  distinction  of  the  carbonates  among  themselves 
and  from  other  similar  minerals,  it  may  be  mentioned  that  in 
the  normal  fresh  occurrences  a  differentiation  of  the  rhombo- 
hedral  carbonates  from  each  other  is  not  possible  in  a  thin  section 
by  simple  optical  investigations.  The  special  reactions  thor- 
oughly discussed  on  page  175  must  be  used  here.  The  same  is 
true  for  conchite  and  the  orthorhombic  series  of  carbonates  of 
which  only  aragonite  occurs  as  a  constituent  of  rocks  and  is 
found  but  rarely.  It  almost  always  forms  fine  fibrous  aggre- 
gates, which  cause  great  difficulty  in  the  determination  of  the 
optical  properties  even  in  convergent  polarized  light.  The  car- 
bonates often  appear  very  similar  to  titanite,  but  observation  of 
the  index  of  refraction  always  gives  a  positive  distinction.  The 
carbonates  can  always  be  positively  determined  in  thin  sections 
by  the  effervescence  with  cold  or  warm  acid. 

Eudialyte  (Eucolite)  (5) 

Eudialyte  forms  rounded  crystals  or  irregular  grains  and  can  always  be 
recognized  macroscopically  by  its  reddish  color.  It  is  usually  colorless  in 
thin  section.  The  optical  character  is  variable  in  one  and  the  same  cross 
section.  Parts  that  are  weakly  doubly  refracting  and  optically  positive, 
alternate  with  those  that  are  isotropic  or  have  a  negative  double  refraction, 
eucolite.  These  sections  do  not  show  anomalous  interference  colors.  The 
members  that  are  optically  negative  are  often  distinctly  colored  in  thin 
section  and  show  weak  absorption,  &>>  e.  The  mineral  has  only  been  found 
in  nepheline  syenite.  It  can  be  distinguished  from  apatite  by  the  fact  that 
the  eudialyte  grains  are  always  considerably  larger. 

Scapolite  Group  (5) 

The  scapolites  form  a-n  isomorphous  series  with  quite  variable 
optical  properties.  The  calcium-aluminium  silicate,  meionite, 
forms  one  end  member  and  a  sodium-aluminium  silicate, 
marialite,  the  other.  The  former  has  higher  indices  and 
double  refraction.  Dipyre,  couseranite,  etc.,  are  intermediate 
members  but  they  are  all  referred  to  in  general  as  scapolite 
because  the  distribution  of  the  various  members  has  been  very 
little  studied. 

The  scapolites  show  crystal  form  only  in  contact-metamor- 
phosed limestones.  In  them,  prismatic  crystals  are  macro- 
scopically visible,  but  in  spite  of  that,  they  are  difficult  to 
recognize;  see  cross  sections  b  and  d,  Fig.  237,  page  225.  Other- 
wise only  granular  or  columnar  aggregates  are  observed  in  which 


DESCRIPTIVE  SECTION  243 

the  cleavage  sometimes  appears  distinctly.  Alteration  to 
micaceous  minerals  with  characteristic  mesh  structure  is  frequent 
as  is  also  alteration  to  homogeneous  individuals  of  colorless 
chlorite — leuchtenbergite.  Special  reactions  for  scapolite,  page 
174,  can  be  made  only  on  fresh  material,  because  the  content 
of  chlorine  is  lost  in  the  process  of  weathering.  The  usual 
occurrence  of  these  minerals  is  in  contact  rocks  and  they  are 
often  so  filled  with  carbonaceous  inclusions  that  they  are  non- 
transparent.  By  metamorphism  black  crystals  may  be  devel- 
oped with  an  appearance  similar  to  that  of  chiastolite,  couser- 
anite,  or  white  knots  with  excellent  sieve  structure  may  be 
formed.  If  alteration  phenomena  also  appear,  it  is  very  difficult 
to  recognize  the  mineral.  It  occurs,  furthermore,  as  granular 
aggregates  in  metamorphosed  diabases  and  gabbros,  especially 
in  the  vicinity  of  the  Norwegian  apatite  dikes.  It  is  likewise 
found  in  various  rocks  called  scapolite  gneiss  but  they  do  not 
have  the  composition  nor  the  geological  significance  of  a  gneiss. 
They  are  mostly  normal  contact  rocks.  Scapolite  often  plays  a 
r61e  analogous  to  that  of  tourmaline  in  the  contact  formation 
of  granites  and  particularly  in  that  of  basic  eruptive  rocks. 
Its  lack  of  characteristic  optical  properties  obscures  an  accu- 
rate conception  of  its  distribution  which  is  undoubtedly  not 
unimportant. 

The  scapolites  are  most  frequently  confused  with  quartz  and 
feldspar.  The  positive  optical  character  of  quartz  and  the 
biaxial  property  of  feldspar  distinguish  them.  Cordierite 
appears  quite  similar  to  the  members  with  low  double  refraction 
but  it  has  no  cleavage  in  addition  to  being  biaxial.  The  special 
reactions  referred  to  above  are  the  safest  since  the  optical 
properties  are  so  variable. 

Alunite  (5) 

Alunite  forms  cube-like  rhombohedrons  and  occasionally  basal  tabular 
crystals  and  flaky  aggregates.  The  cleavage  appears  distinctly  both  macro- 
scopically  and  microscopically.  It  is  only  found  as  an  alteration  product  of 
acid  extrusive  rocks  which  have  been  subjected  to  the  action  of  solfataras. 
It  is  distinguished  from  diaspore,  which  accompanies  it,  by  its  lower  indices 
of  refraction,  and  from  quartz  by  the  cleavage  and  high  double  refraction, 
and  this  latter  property  also  distinguishes  it  from  the  feldspars. 

Beryl  (5) 

Beryl  is  a  rare  constituent  of  certain  granites  and  their  contact  formations, 
and  occurs  in  poorly  bounded  prismatic  crystals.  It  is  more  frequent  in 


244 


PETROGRAPHIC  METHODS 


large,  clear  or  clouded  crystals  in  pegmatites.  It  is  usually  visible  macro- 
scopically  and  is  generally  sky  blue  in  the  eruptive  rocks,  and  sap  green — 
emerald — in  contact  formations.  It  is  sometimes  light  blue  under  the 
microscope.  However,  it  is  usually  quite  difficult  to  determine  the  poorly 
denned  individuals  with  any  degree  of  accuracy,  and  principally,  because  of 
its  similarity  to  quartz,  it  is  generally  overlooked,  if  it  only  occurs  in  micro- 
scopic individuals.  It  is  distinguished  from  quartz  by  the  negative  charac- 
ter of  the  double  refraction. 

Brucite  (5) 

Brucite  is  rare  but  is  found  in  micaceous  flaky  aggregates  in  altered  rocks 
rich  in  magnesium.  It  is  characterized  by  its  tombac  brown,  anomalous 
interference  colors,  which  indicate  a  very  low  double  refraction  that  approx- 
imates that  of  chlorite.  It  can  often  only  be  distinguished  from  chlorite 
by  treatment  with  silver  nitrate  when  the  brucite  becomes  colored  deep 
brown.  Its  radial,  scaly  pseudomorphs  after  periclase  in  contact  lime- 
stones— predazzite — and  its  coarse  flaky  occurrence  in  crevices  in  serpentine 
are  especially  noteworthy.  It  is  distinguished  from  the  micas  by  the  nega- 
tive character  of  the  principal  zone  and  its  easy  solubility. 

Quartz,  Chalcedony  and  Tridymite  (5) 

Quartz  is  one  of  the  most  important  rock-forming  mineralsr 
It  plays  an  important  r61e  in  all  groups  of  rocks.  If  it  has 
crystal  form  it  is  the  hexagonal  bipyramid  alone,  Fig.  251,  os 
in  combination  with  a  very  subordinate  prism.  The  crystal. 


FIG.  251.— Quartz. 


FIG.  252.— Quartz  Section  Parallel 
to  the  Principal  Axis. 


are  originally  sharply  developed  but  the  edges  and  corners  are 
often  considerably  rounded  and  the  faces  greatly  corroded 
forming  irregular,  tubular  indentations  now  filled  with  the  rock 
mass,  Fig.  252  and  Fig.  199,  page  189.  Sections  parallel  to  the 
principal  axis  have  rounded,  rhombic  outlines  with  an  angle 
of  about  100°.  Perpendicular  to  the  principal  axis  the  sections 
are  six-sided,  Fig.  237e,  page  225.  Quartz  is  found  principally 


DESCRIPTIVE  SECTION 


245 


with  this  development  in  quartz  porphyry  and  rhyolite,  and  it 
contains  a  few  glass  inclusions  filling  negative  crystals.  The 
same  development  is  also  seen,  but  less  distinctly  and  more 
rounded,  in  aplites  and  dike  granites,  in  certain  binary  granites 
and  granulites,  Fig.  253,  and  likewise  in  lime-mica  schists. 

Rounded  crystals  of  quartz  are  found  in  basic  eruptive  rocks 
and  they  may  be  present  as  numerous  minute  individuals  pene- 
trating the  basic  constituents,  Fig.  209,  page  194,  especially  in 
certain  gabbros,  or  they  may  be  in  large  shattered  individuals 
surrounded  by  a  border  of  hornblende  or  augite  needles  arranged 
radially  to  the  quartz — quartz-augen,  Fig.  208,  page  193.  These 
are  seen  in  lamprophyres  and  diabases.  In  the  latter  case  the 
origin  of  the  quartz  can  nearly  always  be  distinctly  traced  to 


FIG.  253. — Granulitic  (Aplitic)  Structure. 
Granulite,  Curunegala,  Ceylon. 


FIG.  254. — Dentated  Quartz.     Perosa, 
Cottic  Alps. 


the  shattered  neighboring  rock.  Most  frequently  all  indications 
of  crystal  form  are  lacking  in  the  quartz.  In  most  granites, 
syenites,  diorites,  etc.,  it  fills  the  interstices  between  the  other 
constituents,  Fig.  180,  page  182.  Minute  liquid  inclusions, 
arranged  in  rows,  are  especially  common  in  this  type.  They 
penetrate  through  the  sections  as  cloudy  bands  and  pass  from 
one  grain  over  into  the  neighboring  one  unchanged.  They  are 
the  cause  of  the  clouded,  milk-white  color  of  certain  occurrences 
of  quartz.  Others  contain  abundant  microscopic  needles, 
apparently  rutile,  and  still  others  are  colored  red  by  small 
flakes  of  iron  oxide. 

In  contact  rocks  quartz  generally  forms  an  uniform  mosaic  in 


246  PETROGRAPHIC  METHODS 

which  the  grains  may  simply  lie  in  contact  with  each  other,  Fig. 
184,  page  184,  or  they  may  be  intensely  dovetailed  into  one  an- 
other, Fig.  254.  Again  they  may  form  suture  joints,  but  not 
grow  together  as  in  itacolumite,  Fig.  255.  Helicoidal  structure 
is  found  in  the  quartz  aggregates  in  contact  rocks.  Lines  of 
inclusions  of  carbonaceous  substance  or  sillimanite,  etc.,  corre- 
sponding to  the  original  schistosity  of  the  rock,  wind  about 
through  the  aggregates. 

Quartz  does  not  suffer  any  alteration  by  weathering  and, 
therefore,  occurs  always  entirely  fresh  in  secondary  deposits. 
In  clastic  rocks  it  is  sometimes  angular,  Fig.  256,  and  sometimes 
rounded.  Frequently  it  is  observed,  that  the  fragments  have 


FIG.  255. — Itacolumite.     Ouro  Preto,  FIG.  256. — Clastic  Structure.     Sandstone, 

Brazil.  Schramberg,  Black  Forest. 

grown  into  crystals  by  the  addition  of  secondary  silica — crystal 
sandstone,  Fig.  257.  In  rocks  containing  but  little  quartz,  the 
mineral  is  often  found  in  well  developed  short  prismatic  crystals. 
These  may  be  homogeneous  or  they  may  be  brownish,  due  to 
organic  substance — stink  quartz.  They  may  be  red  to  yellow, 
colored  by  iron  hydroxide,  or  they  may  be  colorless,  clouded,  or 
limpid.  These  occurrences  are  undoubtedly  of  secondary  origin, 
formed  in  place,  and,  for  the  greater  part  at  least,  have  been 
derived  from  the  siliceous  portion  of  organic  skeletons  which  are 
known  to  be  very  soluble.  Quartz  also  occurs  as  a  secondary 
formation  in  granular  or  radial,  columnar  aggregates.  It  may 
be  in  pseudomorphs  after  other  minerals  in  certain  eruptive 
rocks  and  their  tuffs,  or  it  may  be  a  siliceous  cement  as  in  crystal 
sandstone,  where  it  occurs  as  an  enlargement  of  the  original 


DESCRIPTIVE  SECTION 


247 


clastic  grains,  or  again  it  may  occur  in  dense  aggregates  in  sili- 
cified  tuffs  cementing  the  rock  constituents.  In  the  last  case, 
however,  quartz  is  frequently  replaced  by  other  modifications 
of  silica.  In  fritted  sandstone  it  often  has  properties  resembling 
perlite.  Inclusions  in  it  are  fused  to  glass.  Quartz  is  extremely 
brittle  and  is,  therefore,  unlike  any  other  mineral,  an  indicator 
of  the  degree  of  dynamic  action  that  has  modified  the  rock. 
Under  the  influence  of  pressure,  the  appearance  of  cataclastic 
or  mortar  structure,  Fig.  203,  page  191,  is  most  distinctly  devel- 
oped. It  frequently  becomes  biuxial  under  these  conditions. 

Quartz   is   characterized    macroscopically    by    its    concoidal 
fracture  with  somewhat  of  a  greasy  luster  and  its  great  hardness. 


FIG.  257.— Clastic  Structure  Crystal  Sand- 
stone.    Erbach,  Odenwald. 


FIG.  258. — Micropegmatite. 


In  rocks  quartz  is  generally  light  smoky  gray,  but  occasionally 
due  to  inclusions  of  (1)  hematite  flakes  it  is  red,  or  of  (2)  mag- 
netite dust  grayish-blue,  or  of  (3)  chlorite  or  hornblende  green. 
A  light  blue  color,  which  sometimes  occurs,  cannot  be  explained 
by  microscopic  investigation.  Under  the  microscope  quartz 
shows  an  irregular  outline  and  is  always  colorless.  Twinning, 
which  is  also  common  in  rock-forming  quartz,  cannot  be  recog- 
nized in  thin  section.  Graphic  intergrowths  of  microscopic 
dimensions  of  quartz  and  orthoclase  are  very  widespread, 
particularly  in  the  ground  mass  of  porphyric  rocks,  Fig.  258. 
They  are  called  micropegmatites.  Sharp,  angular  sections  of 
clear  quartz  are  sharply  delineated  against  the  clouded  feldspar, 
presenting  a  hieroglyphic  appearance.  Frequently  the  inter- 
growth  becomes  more  and  more  indistinct  and  passes  over  into 


248  PETROGRAPHIC  METHODS 

an  irregular  fibrous  formation,  granophyre,  inclining  to  genuine 
spherulites,  Fig.  189,  page  186.  It  may  become  still  more  irreg- 
ular and  so  fine  that  it  has  no  effect  upon  polarized  light  what- 
ever, microfelsite.  An  intergrowth  similar  to  the  micropegmatite 
occurs  in  which  the  parallel  individuals  of  quartz  penetrating  the 
feldspar  have  worm-like  sections — quartz  vermicide — myrmecitic 
intergrowth,  Fig.  259.  The  feldspar  is  plagioclase.  Both  kinds 
of  intergrowths  are  typical  structures  for  eruptive  rocks. 

Quartz  is  easily  confused  with  several  other  minerals,  which 
like  it,  are  colorless  and  have  a  low  double  -refraction.  It  is 

most  similar  to  nepheline, 
which,  however,  has  a  lower 
double  refraction  and  is  soluble 
in  hydrochloric  acid.  It  re- 
sembles optically  negative  beryl 
and  the  various  zeolites,  which 
can  be  distinguished  from  it 
by  cleavage,  solubility,  and  in 
many  instances  the  biaxial  be- 
havior. Scapolite  is  distin- 
guished by  its  negative  double 
refraction  and  cleavage,  and 
cordierite  by  its  biaxial  proper- 

FIG.  259.— Quartz  Vermicule.  .  r      r    . 

ties    and    frequent    pleochroic 

halos.  The  feldspars  can  be  distinguished  in  general  by  cleavage, 
indices  of  refraction,  and  biaxial  behavior.  This  last  property 
becomes  the  most  important  earmark  for  plagioclase  in  contact 
rocks  where  cleavage,  twinning,  etc.,  are  lacking.  In  order  to 
establish  its  distribution  compared  with  these  minerals,  the 
special  reactions  cited  on  page  174  can  be  employed  to  best 
advantage. 

A  large  number  of  fibrous,  siliceous  minerals,  occurring  in  radial  aggre- 
gates or  having  an  onion-like  structure  or  both  of  these  together  and 
penetrated  by  opal,  have  been  shown  to  be  rock-forming  minerals.  They 
are  found  as  concretionary  masses  in  crevices  and  air  holes  in  eruptive 
rocks,  as  a  cement  in  their  tuffs,  and  as  a  cementing  material  in  sediments. 
Chalcedony  is  the  best  characterized  member  of  this  group.  It  has  a  some- 
what stronger  double  refraction  and  negative  principal  zone,  and  the  acute 
bisectrix  of  a  very  small  optic  angle  lies  perpendicular  to  this  zone.  Quartz- 
ine  and  lussatite  are  similar  to  it.  They  are  biaxial  with  a  small  optic 
angle,  but  are  positive  and  have  positive  principal  zones.  A  variety  known 
as  lutezite  is  distinguished  from  the  others  by  an  oblique  extinction  of  the 


DESCRIPTIVE  SECTION  249 

fibers,  while  pseudochalcedony  is  distinguished  by  the  negative  character  of 
the  principal  zone  and  of  the  mineral.  Distinction  from  zeolites  similar 
to  it  is  only  possible  chemically. 

Tridymite  is  more  characteristic.  Very  small  tabular  crystals  with  an 
hexagonal  outline  are  clustered  together  and  the  little  plates  overlap  each 
other  like  shingles  on  a  roof.  It  shows  division  into  segments  in  polarized 
light,  and  in  convergent  polarized  light  it  gives  a  badly  distorted,  biaxial 
interference  figure.  The  very  low  index  of  refraction  is  its  most  distinctive 
feature,  causing  it  to  appear  in  thin  section  with  decided  relief.  It  occurs 
particularly  in  acid  eruptive  rocks,  rhyolite  and  trachyte,  but  not  as  a 
primary  constituent.  It  is  of  secondary  origin  produced  by  fumaroles  and 
is,  therefore,  not  uniformly  distributed,  but  is  segregated  in  individual 
clusters.  It  is  also  found  in  a  few  fritted  sandstones. 

Nepheline  (6) 

Nepheline  is  never  found  in  association  with  quartz  and  it 
occurs  entirely  in  the  basic  soda  rocks  of  the  series  frorn  nepheline 
syenite  to  theralite,  and  from  phonolite  to  basalt.  Two  types 
can  be  distinguished.  The  one  is  fresh,  has  a  vitreous  luster  and 
is  limpid  in  thin  section.  It  is  called  nepheline.  The  other  is 
macroscopically  reddish  or  greenish  with  a  greasy  luster  and  it 
is  called  elaeolite.  The  latter  is  distinguished  under  the  micro- 
scope by  being  less  fresh.  It  is  filled  with  inclusions  and  de- 
composition products  of  various  sorts  but  it  is  by  no  means  to  be 
considered  as  an  independent  mineral  species.  In 
coarse  granular  rocks  the  mineral  can  be  distinctly 
seen  macroscopically  in  the  last  mentioned  form  and 
the  same  variety  is  also  observed  by  the  naked  eye 
in  nepheline  porphyry.  On  the  other  hand,  the  fresh 
form  is  not  so  readily  observed  partly  because  the 
individuals  are  smaller  and  partly  because  they  are 
clear,  colorless,  and  transparent.  Rocks,  which  con- 
tain nepheline  in  fine  particles,  frequently  show  a  characteristic 
greasy  luster. 

When  nepheline  shows  distinct  crystal  form  its  habit  is  short, 
thick  prismatic,  Fig.  260.  Its  cross  sections  are  short  rectangu- 
lar to  six-sided,  Fig.  237  e,  page  225.  Frequently  inclusions  of 
pyroxene  needles  are  arranged  in  a  zonal  manner.  It  is  usually 
without  distinct  cleavage.  Even  the  variety  known  as  elaeolite 
forms  such  individuals  in  rocks  rich  in  nepheline.  Characteristic 
for  the  shapeless  particles  of  elaeolite  are  the  irregular,  clouded 
bands  of  decomposition  products,  and  the  inclusions  of  scales 
of  brownish-red  ferric  hydroxide  or  of  needles  of  aegirine.  Like- 


250  PETROGRAPHIC  METHODS 

wise,  the  large  grains  of  the  clear,  transparent  type  are  easily 
recognized  by  the  index  of  refraction  which  agrees  almost  ex- 
actly with  that  of  Canada  balsam.  However,  when  the  dimen- 
sions are  very  small,  as  in  numerous  nepheline  basalts,  or  finally, 
when  it  occurs  only  as  a  fine  cement  of  colorless  substance  be- 
tween the  other  constituents,  it  can  only  be  identified  with  an 
approximate  degree  of  accuracy  by  gelatinization  with  hydro- 
chloric acid  and  the  formation  of  cubes  of  salt.  It  can  only 
be  positively  distinguished  from  zeolites  and  rock  glass,  if  in 
addition  to  this  the  etched  grains  show  cross  sections  of  a  typical 
form  after  they  have  been  stained. 

The  fact  that  nepheline  is  confined  to  the  soda  rocks  helps 
to  make  it  more  easily  recognized  because  it  is  almost  always 
accompanied  by  bisilicates  rich  in  sodium  and,  when  these  are 
present,  it  .is  always  well  to  look  for  nepheline.  Its  similarity  to 
the  zeolites  must,  however,  not  be  lost  sight  of.  Numerous 
colorless  minerals  have  been  confused  with  nepheline.  Apatite 
is  distinguished  by  the  higher  indices  of  refraction,  quartz  and 
cordierite  by  the  higher  double  refraction,  scapolite  by  both  of 
these  properties,  and  sanidine  by  a  lower  index  of  refraction. 

Nepheline  is  one  of  -the  most  easily  attacked  of  the  rock  con- 
stituents. Alteration  into  zeolites  such  as  analcite,  hydro- 
nephelite,  and  natrolite  is  especially  common — Spreustein.  In 
certain  rocks  pseudomorphs  of  matted  mica  after  nepheline  are 
found  and  these  have  received  the  name  liebenerite  or  gieseckite. 

Apophyllite  (6) 

Apophyllite  is  rare  and  like  all  the  zeolites  is  only  known  as  a  secondary 
product  mostly  in  basic  eruptive  rocks.  The  mineral  is  characterized  in 
thin  section  by  an  extremely  low  double  refraction,  the  occurrence  of  anomal- 
ous interference  colors,  and  a  very  perfect  cleavage.  It  is  a  very  rare 
mineral  as  a  rock  constituent. 

Chabazite  (6) 

Chabazite  is  also  found  among  the  zeolitic  decomposition  products  of 
silicates  containing  lime.  It  occurs  in  cavities  and  crevices  in  basic  eruptive 
rocks  and  is  found  often  in  splendidly  developed  limpid  crystals.  It  occurs 
as  a  rock  constituent  in  the  same  kind  of  rocks,  but  is  usually  poorly  developed 
and  difficult  to  distinguish  from  the  other  zeolites. 

Cancrinite  (6) 

Cancrinite  is  confined  to  the  soda  rocks.  It  is  sometimes  associated  with 
nepheline  after  which  it  also  forms  pseudomorphs.  It  sometimes  replaces 


DESCRIPTIVE  SECTION  251 

nepheline.  It  appears  macroscopically  in  poorly  bounded  grains  and  scaly 
aggregates  with  a  yellow  to  reddish  color,  and  is  distinguished  by  its  good 
prismatic  cleavage.  Recently,  rhombohedral  crystals  have  been  observed 
in  tephrites.  Under  the  microscope  its  individuals  are  colorless  and  have 
inclusions  of  iron  hydroxides,  etc. 

The  mineral  is  easily  distinguished  from  all  other  rock  constituents  by 
its  low  indices  of  refraction  and  brilliant  interference  colors.  The  identity  of 
the  mineral  can  also  be  proved  by  treating  the  section  under  the  microscope 
with  warm  hydrochloric  acid,  when  the  development  of  small  bubbles  of 
carbon  dioxide  can  be  observed.  If  the  preparation  is  heated,  the  mineral 
becomes  cloudy.  Alteration  is  the  same  as  in  nepheline.  Cancrinite  fuses 
before  the  blowpipe  with  intumescence  to  a  vesicular  glass. 

Hydronephelite  (Ranite)  (6) 

Hydronephelite  and  ranite,  which  is  distinguished  from  it  by  a  small 
content  of  lime,  are  the  most  frequent  alteration  products  of  nepheline. 
They  are  observed  in  confused  flaky  aggregates  in  pseudomorphs  after 
nepheline — Spreustein.  They  can  only  be  recognized  with  any  degree  of 
accuracy  under  the  microscope  in  sections  perpendicular  to  the  c  axis  in 
convergent  polarized  light,  and  in  all  cases  they  are  difficult  to  distinguish 
from  the  other  zeolites,  which  are  associated  with  them  in  the  composition 
of  spreustein. 

4.  Biaxial  Minerals 

When  the  rock-forming  minerals  of  the  orthorhombic,  monoclinic  and 
triclinic  crystal  systems  show  distinct  crystal  form  they  possess  prismatic  or 
tabular  development.  Pyramidal  forms  alone  rarely  occur,  but  they  are 
frequent  as  terminations  on  prismatic  crystals.  Pyramid  faces  also  have 
no  significance  as  cleavage  directions.  Fig.  261  gives  the  principal  types 
of  cross  sections  observed. 

In  the  orthorhombic  system,  sections  parallel  to  the  a  and  c  axes  are  more 
or  less  lath-shaped  with  a  square  or  a  domatic  end.  Sections  parallel  to  a 
macropinacoid  are  quadratic  when  there  are  but  two  end  faces,  and  rhombic 
when  the  development  is  that  of  a  prismatic  form.  When  both  of  these 
types  of  forms  occur  in  combination,  the  sections  are  six-  to  eight-sided.  It 
is  significant  that  the  prism  angle  of  numerous  orthorhombic  minerals,  and 
the  same  is  true  for  monoclinic  and  triclinic  crystals,  approaches  90°  or 
120°,  and  their  cross  sections  are  similar  to  those  of  tetragonal  and  hexago- 
nal minerals.  Quite  frequently  crystals  with  a  lower  symmetry  are  observed 
simulating  the  symmetry  of  a  higher  class.  In  the  lath-shaped  cross 
sections  of  the  orthorhombic  crystals  the  extinction  is  usually  parallel  and 
perpendicular  to  the  edges,  and  the  same  is  true  for  the  sections  across  the 
front  of  the  crystal,  if  its  form  is  determined  by  the  basal  pinacoidal  faces. 
If  a  prismatic  form  predominates  in  such  a  section,  the  extinction  is  sym- 
metrical. 

Monoclinic  minerals  are  sometimes  prismatic  parallel  to  the  axis  of 
symmetry  or  in  the  direction  perpendicular  to  it.  Sometimes  they  are 


252 


PETROGRAPHIC  METHODS 


tabular  parallel  to  the  basal  pinacoid  or  the  plane  of  symmetry,  and  occa- 
sionally to  the  macropinacoid.  In  the  prismatic  type  all  the  sections  with 
parallel  extinction  are  lath-shaped,  while  those  with  oblique  extinction  are 
much  shorter  and  have  four-,  six-  or  eight-sided  outlines.  If  the  prism  axis 
is  perpendicular  to  the  direction  of  the  axis  of  symmetry,  the  sections  with 
parallel  extinction  may  show  all  of  the  forms  referred  to  in  the  orthorhombic 
crystals,  while  the  sections  with  oblique  extinction  appear  elongated  and 
have  unsymmetrical  end  faces.  Minerals  in  the  monoclinic  system,  devel- 
oped tabular  parallel  to  the  base,  have  lath-shaped  cross  sections,  which 
may  show  parallel  or  oblique  extinction,  while  sections  parallel  to  the  base 


FIG.  261. — Principal  Cross  Sections  of  Biaxial  Minerals, 

are  nearly  always  regular  six-sided.  Among  the  minerals  on  which  the 
plane  of  symmetry  predominates,  all  sections  with  parallel  extinction  are 
lath-shaped,  while  those  with  oblique  extinction  are  rhombic  or  irregular 
six-sided. 

The  triclinic  minerals,  which  are  not  abundant  as  rock  constituents,  are 
very  similar  to  the  monoclinic  .in  form.  Generally  it  is  quite  difficult  to 
establish  the  membership  of  the  variously  orientated  cross  sections  in  the 
triclinic  system. 

Sections  of  a  biaxial  mineral  parallel  to  the  plane  of  the  optic  axes  give 
the  highest  interference  colors  of  all  sections.  Such  a  cross  section  must  be 
sought  to  determine  the  value  of  the  double  refraction  by  means  of  the 
interference  colors,  and  one  can  be  assured  of  the  proper  orientation  by  the 
behavior  in  convergent  polarized  light  (see  Part  I,  page  118) .  It  is  also  useful 
to  find  a  cross  section  parallel  to  the  plane  of  the  optic  axes  to  determine  the 
directions  of  extinction  of  monoclinic  minerals,  because  in  many  monoclinic 
substances  the  axial  plane  lies  in  the  plane  of  symmetry,  and  it  is  in  this 
latter  plane  that  the  extinction  angle  must  be  determined.  Sections  of 
biaxial  minerals  perpendicular  to  one  of  the  bisectrices  give  interference 
colors  which  under  all  circumstances  are  lower  than  those  of  a  section  paral- 
lel to  the  plane  of  the  optic  axes.  The  colors  in  the  one  case  are  about  half 
as  high  as  in  the  other,  and  they  are  about  the  same  in  both  sections  perpen- 


DESCRIPTIVE  SECTION  253 

dicular  to  the  acute  or  obtuse  bisectrix  if  the  optic  angle  approaches  90°. 
The  interference  color  in  a  section  perpendicular  to  the  obtuse  bisectrix  is 
always  the  higher,  and  it  is  the  more  like  that  of  a  section  parallel  to  the 
optic  plane,  the  smaller  the  acute  optic  angle.  The  interference  color  is 
very  low  in  a  section  perpendicular  to  the  acute  bisectrix  if  the  optic  angle 
is  small,  even  though  the  double  refraction  of  the  mineral  itself  is  high. 
This  phenomenon  is  especially  interesting  in  the  investigation  of  micaceous 
minerals  in  the  form  of  powder.  In  consequence  of  the  perfect  cleavage  only 
small  flakes  are  found  and  these  are  perpendicular  to  the  acute  bisectrix 
of  a  small  optic  angle.  The  best  example  of  this  is  muscovite.  It  is  optic- 
ally negative,  and  in  certain  occurrences,  f—  a.  =  0.042.  Therefore  the  inter- 
ference colors  in  a  section  parallel  to  the  axial  plane  are  quite  brilliant  in  the 
thinnest  slides.  /?  —  a  =  0.039  shows  that  in  a  section  perpendicular  to  the 
obtuse  bisectrix  the  interference  colors  are  quite  the  same  as  in  the  first 
section.  On  the  other  hand  the  double  refraction  in  a  cleavage  plate  is 
quite  different,  7-— /?  =  0.003.  Distinct  interference  colors  appear  only  in 
very  thick  plates. 

Biaxial  minerals  with  a  small  optic  angle  cannot  always  be  positively 
recognized  even  in  sections  perpendicular  to  the  acute  bisectrix,  because  of 
the  frequency  of  optical  anomalies  in  uniaxial  minerals.  It  is  not  possible 
to  make  a  distinction  in  sections  quite  oblique  or  parallel  to  the  acute  bisec- 
trix. It  was  noted  in  Part  I,  page  118,  that  in  minerals  with  a  large  optic 
angle  and  high  indices  of  refraction  the  optic  axes  cannot  be  seen  in  conver- 
gent polarized  light  even  in  sections  perpendicular  to  the  acute  bisectrix, 
with  a  dry  system  of  lenses,  because  the  axes  are  totally  reflected.  The 
determination  of  the  optical  character  of  such  a  mineral  is  only  possible  in 
oblique  sections  according  to  the  method  described  in  Part  I,  page  115. 
Even  this  method  is  not  always  reliable,  especially  when  the  optic  angle  is 
nearly  90°,  as  in  olivine  and  plagioclase.  It  is  quite  evident  that  the  deter- 
mination of  the  optical  properties  is  more  positive  and  can  be  made  in  more 
sections  where  the  optic  angle  is  large  than  where  it  is  small,  because  most  of 
the  sections  in  the  former  case  will  show  a  good  characteristic  interference 
figure. 

The  position  of  the  plane  of  the  optic  axes  relative  to  the  principal  zone  is 
a  valuable  characteristic  in  determining  a  mineral.  The  plane  is  sometimes 
parallel  and  sometimes  perpendicular  to  the  principal  zone. 

The  size  of  the  optic  angle  is  different  for  various  colors.  For  this  reason, 
if  an  interference  figure  is  rotated  into  the  45°  position  with  respect  to  the 
planes  of  vibration  of  the  nicols,  a  fringe  of  color  will  be  seen  around  the 
vertices  of  the  hyperbolae.  It  is  sometimes  yellow  on  the  convex  side  and  blue 
on  the  concave,  and  sometimes  the  reverse.  In  the  first  case  the  optic  angle 
is  greater  for  red  than  for  blue,  and  the  dispersion  formula  is  ^>y.  In 
monoclinic  and  triclinic  minerals,  the  bisectrices  for  various  colors  do  not 
generally  coincide.  Inclined  dispersion  is  the  most  frequent  in  the  mono- 
clinic  rock-forming  minerals  (see  Part  I,  page  111) .  Strong  dispersion  of  the 
optic  axes  or  of  the  bisectrices  becomes  apparent  in  many  sections  even  in 
parallel  polarized  light  by  the  occurrence  of  anomalous  interference  colors. 

When  biaxial  minerals  show  pleochroism  three  color  axes  can  be  distin- 
guished corresponding  to  the  three  principal  vibration  directions.  In  the 


254  PETROGRAPHIC  METHODS 

orthorhombic  system  these  two  sets  of  axes  coincide.  In  the  monoclinic 
system  the  two  color  axes  lying  in  the  plane  of  symmetry  may  be  oblique 
to  the  principal"  vibration  directions,  while  in  the  triclinic  system  there  is  no 
regularity  in  the  position  of  either  set  of  axes.  In  both  cases  the  differences 
are  quite  small.  An  accurate  orientation  of  the  mineral  section  in  conver- 
gent polarized  light  should  always  precede  an  accurate  determination  of  the 
pleochroism. 

Brookite  (7) 

Brookite  occurs  as  a  rock  constituent  in  small  rather  elongated  plates  with 
domatic  terminations,  but  it  is  a  rare  constituent  and  is  always  in  small 
amounts.  It  is  of  secondary  origin  formed  by  the  alteration  of  silicates  con- 
taining titanium,  especially  of  biotite  in  granite  and  quartz  porphyry,  and 
it  is  also  often  found  in  clastic  rocks.  It  is  always  accompanied  by  rutile  and 
anatase.  It  is  brown  and  transparent  under  the  microscope.  It  shows  no 
pleochroism  on  the  broad  face  and  has  an  adamantine  luster  in  reflected 
light.  The  crossed  position  of  the  optic  planes  for  green  and  red  is  charac- 
teristic. It  can  be  noted  by  observing  the  tabular  crystals  in  convergent 
polarized  light.  The  mineral  can  be  determined  by  its  adamantine  luster 
in  reflected  light  and  high  indices  of  refraction  as  well  as  high  double  refrac- 
tion. Observations  in  convergent  light  distinguish  it  from  rutile,  cassi- 
terite  and  pseudobrookite.  It  is  distinguished  from  goethite  principally  by 
chemical  tests. 

Goethite  (Needle  Iron  Ore)  (7) 

A  few  remarks  concerning  goethite  must  be  made  here.  Its  occurrence 
in  thin  needle-like  inclusions  in  various  other  minerals  has  been  proved,  but 
it  is  scarcely  ever  observed  as  a  rock  constituent  on  account  of  its  similarity 
to  rutile,  brookite,  etc.  Fine  fibrous  aggregates  of  velvet  blend  are  easily 
recognized  in  ore  deposits,  but  otherwise  it  can  only  be  positively  determined 
by  chemical  tests. 

The  mineral,  which  was  originally  called  goethite,  is  quite  different  from 
needle  iron  ore  and  it  is  now  called  ruby  mica.  Its  composition  is  the  same 
as  that  of  goethite,  but  it  occurs  in  short,  rhombic,  tabular  crystals  with 
very  perfect  cleavage  parallel  to  the  broad  face  and  perfect  cleavage  per- 
pendicular to  it,  and  in  addition  it  has  a  fibrous  fracture.  It  is  red  to  yel- 
lowish-red pleochroic,  has  high  indices  of  refraction  and  comparatively  low 
double  refraction.  2V  is  not  far  from  90°  for  all  colors  in  the  same  plane. 
Many  of  the  red  inclusions  producing  the  red  color  or  aventurine  chatoyancy 
of  minerals  belong  to  this  type. 

Fine  grained  to  scaly  aggregates  of  brown  iron  hydroxides  with  brilliant 
interference  colors  are  widely  observed  as  pseudomorphs  after  pyrite.  It 
cannot  be  positively  determined  whether  they  belong  to  goethite  or  not. 
The  same  is  true  for  brown  iron  ore — limonite —  2Fe2O3.3H2O,  which  under 
certain  conditions  forms  similar  aggregates.  It  is  fibrous  with  perfect 
cleavage  in  the  direction  of  the  fibers  and  has  a  positive  principal  zone  and 
negative  double  refraction.  It  has  high  indices.  f~  a:  =  0.05.  2V  is  very 
large.  It  is  brown  to  yellow  pleochroic  fc>  c>  0. 


DESCRIPTIVE  SECTION 


255 


Pseudobrookite  (7) 

Small  rectangular  tabular  crystals  of  pseudobrookite  are  but  rarely  found 
in  the  recent  eruptive  rocks  and  their  tuffs.  It  is  quite  similar  to  brookite, 
but  is  usually  deeper  colored  and  is  probably  always  secondary,  formed  by 
fumaroles.  It  is  red,  transparent  under  the  microscope  only  when  it  is 
extremely  thin  and  is  weakly  pleochroic.  It  is  distinguished  from  brookite, 
goethite  and  wurtzite,  which  appear  quite  similar  to  it  in  ordinary  light, 
by  the  depth  of  color  and  observations  in  convergent  light. 

Sulphur  (7) 

Sulphur  as  a  rock  constituent  is  confined  on  the  one  hand  to  volcanic 
rocks  and  is  then  either  a  by-product  in  alumstone  or  a  binding  material 
in  tuffs  of  volcanic  ash.  It  is  difficult  to  recognize  in  the  latter  case,  but  can 
always  be  determined  by  its  ability  to  ignite  and  by  the  odor  given  off  upon 
combustion.  On  the  other  hand,  it  is  found  in  sediments  especially  in 
gypsum  and  in  organic  deposits  in  which  it  is  apparent  even  macroscop- 
ically.  Its  microscopic  distribution  has  been  very  little  investigated. 

Baddeleyite  (7) 

Up  to  the  present  time  baddeleyite  is  only  known  in  basic  granular  soda 
rocks.  The  cross  sections  are  usually  elongated  and  show  twinning  lamina- 
tion. They  are  pleochroic,  being  green  parallel,  and  brownish  perpendicular 
to  the  principal  zone.  It  has  perfect  cleavage  and  is  characterized  beyond 
question  by  its  high  indices  of  refraction  and  double  refraction. 

Titanite  (7) 

Titanite  is  always  only  an  accessory  constituent  of  rocks,  but 
it  may  be  so  concentrated  locally  that  it  assumes  considerable 
importance.  It  is  distributed  throughout  all  rocks  with  the 


FIG.  262.— Envelope  Form 
of  Titanite. 


FIG.  263. — Titanite.     Horizontal  Cross  Section 
through  Fig.  262. 


possible  exception  of  pure  magnesium  silicate  rocks.  It  has  a 
great  tendency  to  be  associated  with  hornblende.  Its  crystallo- 
graphic  habit  is  quite  variable.  In  granite  and  related  rocks 
the  envelope  form,  Fig.  262,  predominates.  It  gives  a  sharp 


256  PETROGRAPHIC  METHODS 

rhombic  cross  section,  Fig.  263,  caused  by  the  predominance  of 
{1123},  2/3  PS.  In  soda  rocks  it  has  sharp  angular  prismatic 
development  with  {01 1 } ,  P  06  predominating.  Individuals  of  the 
first  type  generally  belong  to  grothite.  It  is  macroscopically 
dark  brown  and  is  often  quite  distinctly  colored,  generally 
reddish,  in  thin  section  and  is  pleochroic.  Those  of  the  second 
type  resemble  more  the  yellow  sphene,  which  is  colorless  in  thin 
section.  Twinning  parallel  to  the  basal  pinacoid  is  not  rare. 
It  often  divides  the  rhombic  cross  sections  in  halves  and  it  also 
occurs  in  the  form  of  lamellae. 

The  mineral  is  further  found  in  grains,  which  are  frequently 
elongated  especially  in  schistose  rocks.  Minute  rounded  particles 
may  be  assembled  together,  insect  eggs,  and  the  finer  grained 
these  are  the  more  clouded  they  appear.  Titanite  is  also  wide- 
spread as  a  border  around  other  titanium  minerals  and  as  pseudo- 
morphs  after  them.  It  occurs  especially  with  anatase  as  the 
principal  constituent  of  leucoxene. 

The  cleavage  is  usually  developed  only  in  a  very  few  cracks 
and  it  is  very  characteristic  that  these  are  generally  not  parallel 
to  the  edges  of  the  crystals,  Fig.  263.  The  optical  properties 
vary  within  wide  limits,  but  the  high  indices  of  refraction  and 
double  refraction  together  with  the  strong  dispersion  of  the 
optic  axes  afford  good  marks  of  recognition.  Deep  colored 
varieties  can  be  confused  with  rutile  under  some  conditions  and 
the  colorless  or  light  colored  ones  with  cassiterite  or  xenotime 
from  which  a  distinction  is  very  difficult,  if  it  cannot  be  made 
by  the  interference  figure.  This  cannot  always  be  done  on 
account  of  the  small  optic  angle  of  titanite.  In  this  case  a 
chemical  test  for  calcium,  made  upon  the  isolated  material,  is 
necessary  to  make  a  positive  distinction.  It  is  easily  distin- 
guished from  anatase,  zircon,  epidote  and  monazite  by  the  much 
lower  double  refraction  of  these.  See  page  242  concerning 
confusion  with  calcite. 

Lievrite  (Ilvaite)  (7) 

Lievrite  is  found  in  large  masses  in  some  ore  deposits  and  in  more  isolated 
individuals  in  contact  rocks.  It  forms  pseudomorphs  after  the  bisilicates 
in  soda  rocks.  It  is  also  encountered  with  fine  hair-like  development  in  the 
pores  of  trachytes  and  is  known  as  breislakite.  Its  determination  is  made 
very  difficult  by  the  fact  that  it  is  almost  entirely  nontransparent  but,  on 
the  other  hand,  it  is  favored  &y  its  solubility  in  hydrochloric  acid  and 
the  micro-chemical  test  for  calcium  as  well  as  by  its  fusibility.  Anig- 
matite  is  similar  to  it,  see  page  294. 


DESCRIPTIVE  SECTION 


257 


(100) 


FIG.  264. — Monazite. 

Section  Parallel  Plane  of 

Symmetry. 


Monazite  (7) 

Monazite  is  a  widespread  constituent  of  granitic  and  syenitic  rocks, 
especially  aplites  and  pegmatites,  and  the  schists  injected  by  them,  but  it 
is  always  only  sporadically  present.  It  occurs  in  tabular  to  prismatic 
crystals,  Fig.  264,  which  have  separated  early  from  the  magma.  It  is  very 
easily  confused  with  zircon  and  epidote  in  thin 
section,  but  observation  with  the  spectroscope 
readily  distinguishes  them.  The  characteristic 
absorption  bands  of  neodymium  and  praseo- 
dymium, page  168,  characterize  monazite.  Cleav- 
age plates  show  an  almost  symmetrical  position 
of  the  positive  bisectrix  of  a  small  optic  angle. 
Since  it  is  quite  resistive  to  weathering,  it  is 
found  also  in  clastic  rocks,  especially  in  sand 
stones,  but  it  is  frequently  clouded  and  generally 
forms  rounded  grains. 

Lavenite  (7) 

When  lavenite  occurs  in  irregular  grains  it  is 
often  difficult  to  distinguish  from  epidote.  In 
most  of  the  prismatic  crystals  the  position  of  the 
optical  plane  parallel  to  the  principal  zone  can  be 
determined.  It  is  reddish-brown  or  yellow  macro- 
scopically  and  in  thin  section  it  is  quite  distinctly 

colored.     Twinning  lamellae  parallel  to  {100}  are  frequent.     One  of  the 
axes  emerges  from  a  cleavage  plate  obliquely.     It  is  always  fresh. 

Woehlerite  is  very  much  like  lavenite.  It  is  also  monoclinic,  but  is  some- 
what lighter  colored  and  has  lower  double  refraction.  It  forms  larger 
individuals,  which  are  usually  tabular  parallel  to  {100}  and  have  twinning 
lamination  parallel  to  the  same  form.  The  extinction  angle  is  about  43° 
and  that,  together  with  the  position  of  the  optical  plane  perpendicular  to 
the  plane  of  symmetry,  is  the  distinctive  characteristic.  The  obtuse  bisec- 
trix, which  is  negative,  is  seen  in  laminated  cross  sections.  It  is  more  easily 
attacked  by  hydrochloric  acid  and  is  more  difficultly  fusible  than  lavenite. 
Both  minerals  are  confined  to  the  soda  rocks,  but  are  widely  distributed  in 
them  in  isolated  grains  or  crystals,  accompanied  by  other  zirconium  silicates 
and  violet  fluorite. 

Another  silicate  containing  zirconium  and  titanium,  not  unlike  lavenite, 
has  been  observed  in  jagged  needles  that  are  frequently  greatly  honeycombed 
and  often  twinned,  in  certain  phonolites  and  is  called  hainite.  Monoclinic? 
It  cleaves  parallel  to  a  direction  perpendicular  to  the  positive  bisectrix  of 
a  large  optic  angle  with  strong  dispersion  p  >  v.  Negative  character  of  the 
principal  zone.  Extinction  angle  up  to  16°.  Index  of  refraction  about  1.7. 
f—  a  about  0.012.  Pleochroism  light  yellow  to  colorless. 

Chrysoberyl  (7) 

Chrysoberyl  undoubtedly  plays  no  subordinate  role  among  the  constit- 
uents that  are  constant,  but  are  only  present  hi  extremely  small  amounts  in 
17 


258 


PETROGRAPHIC  METHODS 


acid  eruptive  rocks,  especially  in  aplites,  pegmatites,  and  in  schists  injected 
by  them.  It  is  very  difficult  to  determine  it,  when  it  occurs  in  small  isolated 
crystals  because  of  its  optical  properties  which  are  unusually  variable.  It 
sometimes  has  a  large  optic  angle  with  a  weak  dispersion,  and  often  the  optic 
angle  is  0°  for  red  and  90°  or  over  for  violet.  It  often  shows  anomalous 
interference  colors  in  consequence  of  the  great  dispersion. 

Epidote  Group  (8) 

The  epidote  group  is  one  of  the  most  difficult  to  recognize  of  all 
the  rock-forming  minerals,  partly  because  of  the  great  similarity 
in  the  optical  properties  of  the  various  members,  which  are  quite 
different  chemically  and  cry stallographic ally,  and  partly  because 
of  the  enormous  differences  displayed  by 
compounds,  chemically  very  closely  related 
to  each  other.  Minerals  of  this  group  with 
an  extremely  low  double  refraction  are 
distinguished  from  those  which  show 
medium  to  very  strong  double  refraction. 
There  are  types  in  the  first  class,  which 
show  the  highest  degree  of  anomalous  interference  colors,  and 
others  that  are  scarcely  observable  in  thin  section  and  give  a 
pure  gray  of  the  first  order. 

All  the  minerals  considered  here  are  developed  prismatic  with 
a   tendency   to   a   long,    tabular   growth.     In   the   monoclinic 


FIG.  265. — Epidote. 


FIG.  266.— Section  through 
a  Twin  of  Epidote. 


FIG.  267. — Section  through 
a  Twin  of  Orthite. 


members  the  principal  zone  is  that  of  the  axis  of  symmetry, 
Fig.  265,  and  the  orthorhombic  members,  contrary  to  custom, 
are  set  up  accordingly,  so  that  the  principal  zone  is  that  of  the 
transverse  axis.  Sharply  bounded  crystals  are  rare.  The  pre- 
dominating cross  sections  of  the  principal  zone  are  mostly 
rounded  on  the  ends.  Corresponding  to  their  development, 
the  monoclinic  members  show  chiefly  sections  with  parallel 


DESCRIPTIVE  SECTION 


259 


extinction.  In  transverse  sections  it  is  not  easy  to  distinguish 
monoclinic  from  orthorhombic  members.  The  extinction  direc- 
tion is  almost  parallel  to  the  trace  of  the  front  pinacoid  in  many 
epidote  minerals,  which  is  usually  also  the  twinning  plane. 
The  two  halves  or  the  lamellae  of  which  such  crystals  are  con- 
structed extinguish  at  about  the  same  time,  Fig.  266. 

Orthite  (allanite)  shows  a  characteristic  difference  in  this 
respect.  Its  extinction  angle  referred  to  this  direction  is  very 
considerable,  Fig.  267,  and  its  twins  therefore  appear  most  dis- 
tinctly in  polarized  light.  It  is  noteworthy,  however,  that  when 
crystal  form  or  twinning  are  lacking,  and  the  extinction  is  meas- 
ured from  the  cleavage  cracks,  which  are  few  in  number  but  very 
sharp,  both  varieties  show  about  the  same  extinction,  30°. 


FIG.  268. — Zoisite  a, 


FIG.  269. — Zoisite 


Section  Parallel  (010). 

There  is  a  difference  here  in  the  axes  of  elasticity.  In  epidote 
and  clinozoisite  the  axis  of  elasticity  which  forms  this  angle  with 
the  cleavage  direction  is  that  of  the  least  velocity  c,  while  in 
orthite  it  is  the  direction  of  the  fast  ray,  a. 

Figs.  268  to  273  show  the  usual  development  of  the  most  im- 
portant members  of  the  epidote  group  together  with  their  most 
important  optical  properties.  In  the  normal  members  of  the 
series  the  plane  of  the  optic  axes  lies  perpendicular  to  the  direc- 
tion of  elongation.  This  is  one  of  the  most  important  means  of 
distinguishing  epidote  from  the  pyroxenes  and  other  minerals, 
which  otherwise  possess  similar  properties.  In  each  of  the  sub- 
divisions of  the  members  with  low  double  refraction,  there  are 
those  in  which  the  position  of  the  axial  plane  coincides  with  the 
trace  of  the  cleavage.  In  the  first  group  transverse  sections  of 
the  prismatic  individuals  are  parallel  to  the  axial  plane  and, 


260 


PETROGRAPHIC  METHODS 


therefore,  show  the  highest  interference  colors.  In  these  sections 
also,  twinning  appears  most  distinctly.  In  the  other  group  such 
transverse  sections  show  the  obtuse  bisectrix  and  have  medium 
interference  colors. 

The  minerals  of  the  epidote  group  are  very  frequently  inter- 
grown  with  each  other  partly  in  a  very  regular,  zonal  manner,  so 


(001) 


V 

*s* 

\p 

y 

™ 

v  v- 

p^ 

(010) 


(100) 


FIG.  270.— Zoisite  a:  Axial  Plane  Parallel 
Cleavage.  Zoisite  0:  Axial  Plane  Perpen- 
dicular Cleavage.  Section  Parallel  Base. 


(010) 

4 


J»t 


(101) 


FIG.  271. — Clinozoisite  and  Epidote. 
Section  Parallel  (100). 


that  the  content  of  iron,  and  with  it  the  double  refraction,  de- 
creases from  the  center  toward  the  edge,  or  vice  versa.  Alternat- 
ing zones  also  occur.  In  other  cases  the  center  may  be  one  of 
the  zoisites  which  grows  into  a  monoclinic  member  toward  the 
edge.  One  of  the  most  characteristic  features  of  orthite  is  an 


^(001) 


FIG.  272. — Clinozoisite, 


or         >4 

FIG.  273. — Epidote, 


Section  Parallel  Plane  of  Symmetry. 

enveloping  growth  of  Clinozoisite.     In  such  cases  the  following 
parallelism  of  elasticity  axes  is  to  be  noted: 

zoisitea  zoisite/9  Clinozoisite,  epidote,  orthite. 

r  r  a 

P  *  r 

a 


DESCRIPTIVE  SECTION  261 

The  crystallographic  orientation  of  the  members  of  the  group 
referred  to  above  is  taken  from  these  observations.  In  other 
cases  the  intergrowth  is  entirely  irregular  and  the  various 
members  penetrate  each  other.  The  great  differences  in  double 
refraction  of  the  members  thus  intergrown  often  give  rise  to 
the  brilliant  speckled  appearance  of  the  sections  in  polarized 
light,  and  this  does  not  occur  in  any  other  series  of  minerals. 

Zoisite  and  clinozoisite  are  scarcely  ever  distinctly  seen  macro- 
scopically, but  rocks  in  which  they  participate  in  the  composition 
to  a  great  degree  have  a  light  yellowish-  or  grayish-green  tint. 
Epidote  itself  with  its  pistachio  green  color — pistazite — is  often 
seen  macroscopically.  It  is  the  most  frequent  yellow  pigment  of 
crystalline  rocks  and  as  such  is  in  a  fine  state  of  division.  Nor- 
mal zoisite  and  clinozoisite  are  colorless  in  thin  section.  Mono- 
clinic  epidote  rich  in  iron  is  quite  light,  usually  yellowish,  with 
strong  absorption  parallel  to  the  principal  zone.  The  intensity 
of  the  color  and  the  pleochroism  are  increased  by  heating  to 
redness  in  air.  A  deep  brownish-yellow  color  appears  in  an 
unroasted  section  of  the  members  with  the  highest  double  refrac- 
tion and  the  highest  content  of  iron,  but  this  seems  to  be  a  rare 
occurrence.  Similarity  to  lavenite  appears  in  the  most  highly 
colored  members. 

There  is  a  series  of  members  containing  chromium  which  are  macroscopic- 
ally  almost  emerald  green.  In  this  series  chrome  zoisite  and  chrome  epidote  can 
be  distinguished,  and  there  are  also  varieties  corresponding  to  clinozoisite. 
These  minerals  are  deeply  colored  in  thin  section  and  the  pleochroism  (o=  c 
light  green,  B  deep  orange)  can  be  noted  in  occurrences  that  have  such  a  low 
double  refraction  that  they  scarcely  affect  polarized  light.  The  color  and 
strength  of  the  double  refraction  are  in  no  way  related  to  each  other. 

The  whole  series  can  be  distinguished  also  in  the  members  containing 
manganese.  Thulite  corresponds  to  the  zoisites.  It  is  macroscopically 
rose-red  and  has  a  distinct  color  in  thin  section;  a  yellowish,  B  red,  c  rose. 
It  usually  has  anomalous  interference  colors.  A  series  with  varying  inten- 
sity of  color  and  strength  of  double  refraction  leads  to  manganese  epidote — 
piemontite.  It  appears  blackish-red  macroscopically  and  it  is  characterized 
in  thin 'section  by  deep  color  and  strong  pleochroism.  It  is  often  the  cause 
of  the  deep  red  color  of  rocks,  especially  in  altered  porphyrites,  e.g.,  in 
porfido  rosso  antico,  and  in  phyllites.  Next  to  hematite  it  is  the  most  impor- 
tant red  pigment  of  crystalline  rocks.  It  often  appears  not  unlike  dumor- 
tierite,  but  is  distinguished  from  it  by  a  different  optical  orientation. 

Finally  orthite  or  ceriwn  epidote,  allanite,  is  to  be  mentioned.  It  usually 
shows  good  crystal  form  in  the  granular  eruptive  rocks,  and  these  as  well  as 
the  isolated  grains  are  characteristically  enveloped  by  clinozoisite.  It  has 
a  brown,  sometimes  also  blood  red,  color  and  is  strongly  pleochroic  with  a 


262 


PETROGRAPHIC  METHODS 


high  double  refraction,  7-—  a  =  up  to  about  0.3.  Isolated  black  grains  with 
a  pitchy  luster,  surrounded  by  a  red  border,  are  frequently  observed  macro- 
scopically.  It  is  noteworthy  that  these  deep  colored  orthites  are  not 
infrequently  transformed  into  an  amorphous  yellowish  to  red  gum-like 
mass,  a  phenomenon  which  numerous  minerals  containing  cerium  show.  The 
color  of  orthite  is  very  much  lighter  in  the  soda  rocks,  contact  rocks,  and 
injected  schists.  In  thin  section  it  is  light  brownish,  greenish-yellow,  or 
light  violet,  probably  due  to  titanium.  Some  individuals  are  indeed  colorless. 
The  double  refraction  is  decreased  in  such  occurrences  to  about  0.002,  and 
the  index  of  refraction  of  these  lighter  colored  members  also  seems  to  be 
much  lower,  about  1.70  or  less.  The  determination  of  small  prisms  is  thus 
often  extremely  difficult.  They  often  show  normal  interference  colors  and 
are  then  nearly  always  determined  as  apatite.  Sometimes  they  are  charac- 
terized by  brilliant  anomalous  colors. 

There  are  many  reasons  for  classing  these  small  crystals  as  orthite.  Fre- 
quent zonal  growths  with  clinozoisite  point  to  their  membership  in  the  epi- 
dote  group.  The  appearance,  of  pleochroic  halos  is  often  observed  near 
them,  and  this  is  distinctive  for  all  occurrences  of  orthite,  and,  finally,  the 
transverse  sections  nearly  always  show  twinning  with  the  orientation  of 
Fig.  267,  page  258.  Hence  there  can  be  no  doubt  of  their  relation  to  orthite. 
It  may  be  mentioned  that  in  spite  of  the  high  content  of  rare  earths,  spectro- 
scopic  investigation  of  orthite  usually  gives  no  results. 

The  following  table  gives  a  classification  of  the  phenomena  described  and 
can  be  used  to  differentiate  these  minerals. 


Double 
refraction 

Inter- 
ference 
color 

Position  of 
optical 
plane 

Optic 
angle 

Extinction 

Zoisite  a. 

Anomal- 

||   cleavage 

Medium. 

i 

ous. 

and  princi- 

pal zone. 

Zoisite  /?. 

Low. 

Normal. 

1 

Small. 

>  Approxi- 
mately or 

Clinozoi- 
site. 

Strongly 
anomal- 

J_ cleavage 

1  Very 

entirely 
parallel. 

ous. 

i  and  prin- 
cipal zone. 

large. 

> 

Epidote. 

High. 

Normal 

speckled  . 

|  ' 

Orthite. 

Medium 

Anomal- 

Variable. 

Variable. 

Oblique  36°. 

to  low. 

ous   or 

normal. 

DESCRIPTIVE  SECTION  263 

The  great  difficulties  encountered  in  making  an  accurate 
determination  of  the  epidote  minerals  can  be  seen  from  this 
classification.  A  distinction  is  quite  important  where  the  mem- 
bers are  of  considerable  size  and  to  some  extent  well  developed. 
Much  practice  is  required  to  distinguish  the  various  members, 
where  the  minerals  occur  in  dense  aggregates  as  in  the  saussurite, 
which  is  macroscopically  greenish-gray  and  has  a  splintery 
fracture,  or  where  they  are  in  sharply  defined  microlites  grown 
in  plagioclase  as  is  generally  the  case  in  the  central  granite. 
If  cleavage  plates  of  these  minerals  can  be  investigated,  it  is 
noted  that  in  epidote  rich  in  iron  an  axis  emerges  slightly  oblique 
to  the  cleavage  face.  In  clinozoisite  it  is  more  oblique  and 
cleavage  plates  of  the  zoisites  show  no  characteristic  interference 
whatever. 

Distinction  of  the  two  zoisites,  which  are  not  positively  ortho- 
rhombic  but  possibly  crystallize  triclinic,  from  clinozoisite,  which 
is  certainly  monoclinic,  is  made  difficult  by  the  fact  that  the 
latter  has  an  extinction  almost  parallel  to  the  vertical  axis.  The 
degree  of  abnormality  of  the  interference  colors  is  the  safest 
means  of  differentiation.  It  is  pure  gray  in  zoisite  /?,  distinct 
bluish-gray  in  zoisite  a,  and  brilliant  Prussian  blue  in  clino- 
zoisite. Investigation  in  convergent  polarized  light  also  aids 
to  distinguish  them.  Thorough  study  shows  that  clinozoisite 
is  by  far  the  most  widespread  of  the  low  double  refracting  mem- 
bers, which  were  formerly  simply  classed  together  as  zoisite. 
Zoisite  a  is  rather  frequent,  but  zoisite  /?  is  quite  rare  as  an  in- 
dependent rock  constituent. 

Concerning  the  geological  distribution  of  these  minerals,  it 
may  be  said  they  belong  to  the  most  typical  formations  of  con- 
tact rocks  rich  in  aluminium  and  calcium,  and  as  such  are  found 
especially  abundant  in  amphibolites,  chlorite  schists,  green 
schists,  and  eclogites,  in  which  they  are  regularly  mixed  with 
the  other  minerals  or  are  separated  in  lighter  colored  layers. 
They  are  observed  less  frequently  in  mica  schists,  but  in  them 
they  have  a  tendency  to  occur  in  knots  rich  in  graphite,  which 
are  macroscopically  black.  As  already  mentioned  they  form 
a  constituent  of  saussurite,  which  occurs  in  pseudomorphs  after 
plagioclase  rich  in  calcium.  In  it  these  pseudomorphs  are  usually 
very  poorly  developed  and  the  structure  of  the  saussurite  is  so 
matty  that  the  individual  grains  cannot  be  separated  from 
each  other  even  under  the  microscope.  Since  numerous  other 


264 


PETROGRAPHIC  METHODS 


minerals  with  high  indices  and  low  double  refraction,  frequently 
also  with  anomalous  interference  colors,  such  as  vesuvianite, 
garnet,  gehlenite,  prehnite,  etc.,  take  part  in  the  formation  of 
these  aggregates,  the  question  whether  saussurite  really  belongs 
to  the  epidote  minerals  must  remain  unsettled. 

The  low  double  refracting  members  occur  alone  in  some  rocks, 
especially  in  amphibolites  and  saussurites.  In  other  rocks, 
however;  they  are  in  zonal  intergrowth  with  epidote,  which  is 
richer  in  iron  and  has  a  higher  double  refraction.  In  numerous 

chlorite  schists,  eclogites,  etc., 
epidote  entirely  replaces  the 
other  members.  In  lime- 
silicate  fels  rich  in  aluminium, 
lime"  mica  schist,  etc.,  epidote 
is  in  general  more  frequent 
than  the  other  members. 
These  rocks  pass  over  into 
yellowish  epidote  fels  or  epi- 
dosite.  In  the  latter  rock, 
epidote  is  observed  here  and 
there  in  crystals  well  bounded 
on  all  sides,  while  in  other 
occurrences  it,  like  the  others, 
shows  good  faces  at  best  only 
in  the  zone  of  the  axis  of  sym- 
metry. Sections  parallel  to  the  a  axis  generally  appear  as 
rounded  tapering  laths. 

The  widespread  occurrence  of  abundant  sharply  defined 
microlites  of  clinozoisite  in  the  plagioclase  of  the  central  granite 
is  very  noteworthy.  Fresh  glassy  feldspar  consisting  mostly  of 
oligoclase  is  often  so  abundantly  filled  with  these  microlites 
without  any  orientation  that  the  rock  shows  a  yellowish-green 
tint  macroscopically,  and  extremely  thin  sections  are  necessary 
to  make  the  formation  transparent  at  all.  Fig.  274  gives  a 
representation  of  this  phenomenon,  which  differs  greatly  from 
all  forms  of  secondary  development  and  can  only  be  designated 
as  an  original  formation.  It  represents  a  typical  feature  of 
piezocrystallization.  Besides  this,  clinozoisite  and  epidote  are 
encountered  in  much  larger  grains  as  primary  constituents  of 
acid  eruptive  rocks.  They  are  generally  confined  to  the  border 
zone  of  the  intrusive  and  represent  constituents  taken  up  from 


FIG.  274. 

Plagioclase  with  Microlites  of  Clinozoisite. 
Central  Granite,  Mosele,  Zillertal. 


DESCRIPTIVE  SECTION  265 

the  neighboring  rock.  The  zoisites  are  also  found  in  intruding 
aplites  and  pegmatites.  They  occur  in  large  crystals  in  peg- 
matites, which  intrude  the  eclogites  of  the  Fichtelgebirge,  and 
in  microscopic  individuals  in  the  aplites,  which  intrude  the 
amphibolites  of  Mount  Grossevenedig.  They  are  much  more 
frequent  in  the  eruptive  rocks,  where  they  are  secondary.  The 
alteration  of  bisilicates  into  chlorite  is  almost  always  accompanied 
by  the  formation  of  greater  or  smaller  amounts  of  epidote  in 
irregular  grains.  If  this  form  is  but  weakly  colored  it  appears 
similar  to  anatase,  which  occurs  under  like  conditions  but  is 
distinguished  from  the  latter  by  much  lower  indices  of  refraction. 
The  manner  of  occurrence  of  manganese  epidote  has  already 
been  referred  to  and  likewise  that  of  orthite.  Aside  from  the 
extinction  and  the  frequent  occurrence  of  pleochroic  halos  in 
the  neighborhood  of  orthite,  it  is  characterized  by  the  fact  that 
it  is  always  only  a  subordinate  and  purely  accessory  rock 
constituent. 

Minerals  of  the  epidote  group  are  not  infusible  and  the  richer 
they  are  in  iron  the  more  easily  they  fuse.  Orthite  is  distinctly 
attacked  when  fresh  by  hydrochloric  acid,  but  the  other  members 
resist  the  acid  unless  they  have  previously  been  roasted  to  drive 
off  water  and  then  they  gelatinize  readily. 

Confusion  of  epidote  minerals  with  numerous  others  has 
already  been  referred  to  many  times,  thus  with  vesuvianite, 
etc.,  in  saussurite  where  a  separation  is  not  possible.  Light 
colored  epidote  is  often  not  unlike  the  pyroxenes.  They  have 
numerous  cleavage  cracks  parallel  to  which  lies  the  optical  plane, 
while  in  epidote  it  is  perpendicular.  In  lavenite  also  the  optical 
plane  lies  in  the  principal  zone.  Olivine  often  appears  similar 
to  epidote,  but  it  never  has  the  speckled  interference,  colors  and 
generally  has  no  distinct  cleavage  cracks.  It  also  turns  deep 
brown  upon  roasting  and  gelatinizes  readily  with  hydrochloric 
acid.  The  isolated  occurrence  and  spectroscopic  investigation 
distinguish  monazite.  Confusion  of  low  double  refracting 
members  with  apatite  has  been  mentioned  above.  When  no 
morphological  distinction  can  be  found,  apatite  can  be  deter- 
mined by  its  solubility  in  dilute  nitric  acid  and  its  reaction 
for  phosphorus.  Brown  orthite  often  appears  not  unlike  basaltic 
hornblende.  It  shows  a  smaller  difference  in  absorption  than 
does  the  hornblende  and  the  position  of  the  optical  plane  trans- 
verse to  the  principal  zone.  Further  than  this  the  border  of 


266 


PETROGRAPHIC  METHODS 


clinozoisite  and  epidote  around  the  individuals  of  orthite,  which 
are  always  isolated,  is  exceedingly  characteristic. 

Staurolite  (8) 

Staurolite  is  a  typical  mineral  of  the  contact  rocks,  particu- 
larly those  formed  by  piezocontact  metamorphism.  It  is  rarely 
found  in  the  granites,  but  is  found  in  the  clastic  rocks  in  con- 
sequence of  its  great  resistance  to  weathering. 
It  sometimes  occurs  in  quite  large,  well  developed, 
flat  prisms  and  twins,  in  which  the  individuals 
interpenetrate  each  other  at  90°  or  60°,  Fig.  275. 
They  are  dark  brown  in  color.  Sometimes  the 
borders  are  very  irregular.  Smaller  microscopic 

FIG.    275.  ......  11        i  £ 

Staurolite  Twins,  individuals  usually  have  a  much  poorer  form, 
but  in  general  the  prismatic  development  is 
distinct.  The  larger  crystals  are  usually  filled  with  inclu- 
sions like  a  sieve.  They  may  be  penetrated  by  quartz  in  an 
irregular  manner  or  in  a  helicoidal  manner  as  shown  in  Fig. 
276.  Graphite  dust  is  observed  in  them  much  the  same  as  in 
chiastolite.  Microscopic  individuals  tend  to  be  purer,  but  are 
difficult  to  determine  on  account  of  their  poor  development. 


FIG.  276.— Staurolite  Twin  Filled 
with  Quartz  Inclusions. 


FIG    277. 
Staurolite  Cross  Section. 


Sections  corresponding  to  Fig.  277  are  the  most  characteristic 
for  the  mineral.  It  is  distinguished  from  the  yellow  varieties 
of  tourmaline  or  from  colored  epidote  by  the  lower  double 
refraction  and  by  the  orientation  of  the  absorption,  which  is 
strongest  in  the  direction  of  the  principal  zone.  Now  and  then 


DESCRIPTIVE  SECTION  267 

pleochroic  halos  are  present  in  the  mineral.  It  can  be  easily 
isolated  because  of  its  insolubility  even  in  cold  hydrofluoric 
acid.  It  is  strongly  attracted  by  an  electro-magnet  and  is 
infusible  before  the  blowpipe. 

Diaspore  (8) 

Diaspore  has  been  positively  recognized  only  as  a  decomposition  product 
in  completely  altered  rhyolites,  in  the  spreustein  of  nepheline  syenites,  in 
kaolin,  in  emery,  and  in  other  rocks  bearing  corundum.  Its  distribution 
other  than  this  cannot  be  determined.  Small  crystals  of  it  are  rounded 
and  rather  elongated  plates.  Sharp  cracks  occur  parallel  to  the  direction 
of  the  a  axis.  It  is  occasionally  colored  and  is  then  pleochroic.  Its  in- 
solubility allows  it  to  be  isolated.  The  development  and  negative  char- 
acter of  the  principal  zone  distinguish  it  from  sillimanite.  Higher  double 
refraction  and  a  good  cleavage  distinguish  it  from  andalusite;  parallel  ex- 
tinction and  the  lack  of  fibrous  fracture  from  cyanite,  and  the  double  re- 
fraction and  cleavage  from  sapphirine.  It  is  most  similar  to  lawsonite. 
Aside  from  its  indices  and  double  refraction,  it  may  be  distinguished  from 
lawsonite  by  its  inf usibility. 

Cyanite  (Disthene)  (8) 

Cyanite  forms  broad,  tabular  individuals  usually  poorly 
bounded  with  oblique  four-  or  six-sided  cross  sections.  It 
often  occurs  with  a  light  blue  macroscopic  color  and  perfect 
cleavage.  It  is  also  found  in  certain  piezocontact  rocks  in 
radial  aggregates  or  sheaves,  which  are  macroscopic  ally  black 
and  entirely  filled  with  graphite  dust — rhaetizite.  Twins  are 
very  widespread,  the  macropinacoid  being  the  twinning  plane. 
Closely  assembled  cracks  representing  fibrous  fracture  parallel 
to  the  base  are  frequently  observed,  especially  in  bent  sections. 
These  are  perpendicular  to  the  sharp  cracks  representing  cleav- 
age parallel  to  the  macropinacoid  and  to  the  less  regular  cracks 
of  the  cleavage  parallel  to  the  brachypinacoid.  A  light  bluish 
tint  can  frequently  be  discerned  in  thin  sections.  Pleochroism 
can  also  be  recognized  if  some  care  is  exerted.  Deeper  colors 
are  rare  and  a  mottled  appearance  is  characteristic  of  their 
presence. 

Cleavage  plates  parallel  to  the  macropinacoid  are  nearly 
perpendicular  to  the  negative  bisectrix  of  a  large  optic  angle, 
Fig.  278.  The  axial  plane  forms  an  angle  of  about  30°  with  the 
cleavage  cracks  parallel  to  the  brachypinacoid.  This  makes 


268 


PETROGRAPHIC  METHODS 


(001) 


the  mineral  easily  distinguished  from  all  other  rock  constituents. 
Cleavage  plates  from  twins  show  a  brilliant  change  of  color  in 
parallel  polarized  light,  but  no  difference  in  extinction.  The 
fact  that  macropinacoidal  sections  of  twins  show  different  colors 
in  the  two  halves,  but  extinguish  nearly  at  the  same  time,  is  the 
most  characteristic  feature  of  the  mineral  and  prevents  it  from 
being  confused  with  any  other. 

The  principal  field  of  distribution  of  cyanite  is  in  mica  schists 
where  it  is  frequently  extremely  rich  in  inclusions  of  other 
minerals.  It  is  penetrated  by  microlites  of  rutile,  tourmaline, 
and  quartz  grains  in  a  sieve-like  manner. 
It  is  often  so  filled  with  graphite  dust 
that  it  appears  almost  nontransparent. 
Then  its  determination  in  thin  section 
is  extremely  difficult.  In  eruptive  rocks, 
especially  in  cyanite  granulites  and  in 
pegmatites,  it  is  macroscopically  visible 
and  in  thin  section  it  is  poor  in  inclu- 
sions and  often  distinctly  blue.  The 
most  beautiful  occurrences  of  the  mineral 
— paragonite  schists — belong  here.  It  is 
frequently  associated  with  staurolite  in 
such  rocks  and  is  often  in  parallel  growth 
with  it. 

In  thin  section  it  is  quite  similar  to  a 
whole  series  of  minerals,  such  as  sillimanite,  andalusite,  topaz, 
colorless  epidote,  diaspore,  lawsonite,  sapphirine,  serendibite,  and 
finally  light  blue  hornblende.  It  is  distinguished  from  all  of 
them  by  its  fibrous  fracture,  the  oblique  extinction  in  sections 
perpendicular  to  the  negative  bisectrix  or  in  the  cleavage  plates 
and  by  the  nearly  parallel  extinction  of  macropinacoidal  sections 
of  twins. 


FIG.    278. 
Cyanite,  Cleavage  Plate. 


Sapphirine  (8) 

Sapphirine  has  been  positively  determined  only  in  rocks  from  Greenland 
forming  masses  in  gneiss  and  having  the  character  of  contact  rocks.  It 
may  be  found  widely  disseminated  in  the  eclogites.  The  properties  of 
sapphirine  in  thin  section  are  not  very  distinctive.  It  may  be  discovered 
if  it  is  blue  or  bluish-green,  but  its  determination  is  very  difficult  if  it  is 
colorless  or  occurs  as  poorly  bounded  tabular  crystals  in  parallel,  scaly 
aggregates  or  those  which  are  very  imperfectly  radial.  It  must  be  isolated 
with  hydrofluoric  acid  in  such  cases,  but  even  then  it  can  be  identified  only 


DESCRIPTIVE  SECTION  269 

with  difficulty.  It  may  be  confused  with  all  the  minerals  mentioned  under 
cyanite.  Lack  of  cleavage  distinguishes  it  from  most  of  them.  It  is 
distinguished  from  serendibite  by  lack  of  twin  lamination,  from  corundum 
by  its  biaxial  properties,  and  from  cordierite  by  its  much  higher  indices  of 
refraction. 


Serendibite  (8) 

This  mineral  has  only  been  found  in  the  skarn-like  border  between  a 
pegmatite  and  limestone  in  Ceylon.  The  lack  of  cleavage,  typical  twinning 
lamination,  large  extinction,  and  the  pleochroism — from  nearly  colorless 
to  sky  blue  or  indigo — are  sufficient  indication  of  its  presence. 


Prismatine  (Kornerupine)  (8) 

Prismatine  and  kornerupine,  which  is  closely  related  to  it,  are  rare 
minerals  in  pegmatites  and  are  associated  with  sapphirine.  In  thin  section 
they  are  very  similar  to  sillimanite  in  form  and  habit,  but  may  be  distinctly 
separated  from  it  by  the  negative  character  of  the  principal  zone.  They 
are  distinguished  from  apatite  by  higher  double  refraction,  from  tremolite 
and  the  brittle  micas  by  parallel  extinction  in  all  sections,  and  from  anda- 
lusite  by  the  small  optic  angle. 


Astrophyllite  (9) 

Astrophyllite  is  not  rare  in  soda  rocks  as  an  accessory  constituent.  It  is 
macroscopically  visible  in  micaceous  individuals  that  are  brittle  and  bronze 
colored  or  in  star-shaped  groups.  In  thin  section  it  appears  as  laths  with  a 
perfect  cleavage  and  strong  pleochroism.  It  is  distinguished  from  micas 
by  its  high  indices  of  refraction,  from  the  brittle  micas  by  its  high  double 
refraction,  and  from  yellowish-brown  hornblende  by  the  perfect  cleavage, 
which  shows  only  in  one  direction  in  sections  parallel  to  the  front  pinacoid. 


Brittle  Mica  Group  (9) 

Many  micaceous  minerals  are  classed  together  under  the  term 
of  the  brittle  micas.  They  are  distinguished  from  mica  and  other 
flaky,  micaceous  minerals  by  greater  hardness,  decidedly  higher 
indices  of  refraction  and  lower  double  refraction. 

The  following  table  is  arranged  to  show  a  comparison  of  the 
properties  of  these  minerals. 


270 


PETROGRAPHIC  METHODS 


Index  of 
refraction 

Double 
refraction 

Extinction 

Optic  angle 

Astrophyllite.  .  . 

ITTio-Vi 

High. 

Parallel. 

Medium  large. 

Brittle  micas.  .  . 

xiign. 

Medium    to 

Quite  large. 

j 

Oblique. 

Margarite  

< 

^uite  high. 

Low. 

Large. 

Mica  

• 

High. 

} 

Very   small   to 

I  Parallel. 

medium  large. 

Chlorite. 

Low. 

Very  small. 

Clinochlore  

}oh1iniip 

Medium  large. 

Hydrargillite.  .  . 

>  Low. 

1  Small 

Tale 

1 

] 

J 

I  TTio-V» 

I  Parallpl 

Pyrophyllite  

f  fli^u- 

Medium  large. 

Kaolin 

Low 

Oblique. 

Large. 

They  possess  a  distinct  cleavage  parallel  to  the  basal  pinacoid 
and  one  or  more  imperfect  cleavages  transverse  to  that.  The 
optical  properties  of  the  brittle  micas  are  not  very  constant. 
Differences  in  index  of  refraction  are  uncommonly  distinctive 
and  the  differences  in  double  refraction  are  not  small.  The 
optical  character  is  sometimes  positive  and  sometimes  negative. 
The  color  is  likewise  quite  variable.  Some  are  colorless,  while 
others  are  more  or  less  deeply  colored,  and  then  pleochroic.  They 
show  stronger  absorption  of  the  ray  vibrating  perpendicular  to 
the  cleavage,  which  is  uncommon  for  micaceous  minerals. 

Certain  varieties  of  chloritoid  are  undoubtedly  triclinic.  In 
cleavage  plates  of  such  varieties  the  optical  plane  lies  unsym- 
metrical  to  the  percussion  figure.  Other  varieties  approach  the 
hexagonal  system  in  their  behavior.  The  specific  gravity 
varies  within  just  as  wide  limits.  Most  of  the  brittle  micas  are 
not  attacked  by  acids,  but  some  are  comparatively  easily  dis- 
solved— ottrelite.  Chemical  analysis  does  not  point  to  a  common 
formula  for  the  brittle  micas  so  that  the  question  of  the  relation- 


DESCRIPTIVE  SECTION 


271 


ship  of  the  members  must  remain  open.  The  two  members 
given  in  the  tables  represent  the  end  members  of  the  series  with 
ottrelite,  brandisite,  clintonite,  sismondine,  masonite,  seybertite, 
etc.,  arranged  intermediately.  Nothing  is  known  about  the 
relative  delimitations  of  these  in  rocks. 

•The  brittle  micas  are  widely  disseminated  constituents^  in 
contact  rocks,  especially  in  cases  of  piezocontact  metamorphism 
and  are  found  but  rarely  in  the  inner  contact  zone,  e.g.,  in  the 
coarse  crystalline  silicate  fels  in  the  Fassatal  and  in  certain 
eclogites,  etc.  On  the  other  hand,  they  are  the  most  character- 
istic constituents  of  the  outer  contact 
zone  and  their  principal  field  of  dis- 
tribution is  in  such  rocks  as  phyllites. 
They  appear  macroscopically  in  the 


FIG.  279.— Chloritoid,  Cleavage  Plate. 


FIG.  280.— Chloritoid, 
Section  Parallel  Plane  of  Symmetry. 


phyllites  in  elongated  or  disc-shaped  knots — ottrelite — and  occa- 
sionally also  in  sheaf-like  and  radial  aggregates.  They  may  be 
recognized  by  the  greenish-black  color  and  brilliant  luster. 

Abundant  inclusions  are  usually  observed  under  the  micro- 
scope, especially  in  the  knots.  These  are  often  penetrated  with 
quartz  grains,  giving  them  the  sieve  structure  or  filled  with  graph- 
ite dust  until  they  are  nontransparent.  Zonal  structure,  hour- 
glass development,  and  abundant  twinning  lamination  are  often 
observed.  Index  of  refraction,  double  refraction,  and  particu- 
larly the  pleochroism  of  the  larger  individuals,  are  very  important 
properties  but,  nevertheless,  they  are  frequently  confused  with 
cyanite.  The  large  tabular  cross  sections  of  the  latter  show  more 
distinct  cleavage  than  the  basal  sections  of  the  brittle  micas. 
Cyanite  is  also  characterized  by  a  larger  optic  angle.  The  brittle 
micas  are  often  confused  with  bluish-green  hornblende  from 
which  they  can  only  be  distinguished  by  observations  in  con- 
vergent light,  particularly  in  cleavage  plates,  Fig.  279. 


272 


PETROGRAPHIC  METHODS 


The  brittle  micas,  as  microscopic  constituents,  are  much  more 
widespread  than  the  larger  individuals.  The  development  is 
such  that  they  can  hardly  be  determined  positively.  The  nor- 
mal occurrence  is  in  small  irregular  particles,  grouped  in  radial 
aggregates,  in  which  the  graphite  content  of  the  rock  is  so  con- 
centrated that  they  are  nontransparent.  These  individuals, 
which  often  cannot  be  seen  at  all  until  after  they  have  been 
roasted  in  an  oxidizing  flame,  are  quite  light  colored  and  some- 
times entirely  colorless.  Twinning  and  cleavage  can  scarcely 
be  recognized  in  them.  They  are  constituents  which  can  be 
determined  more  by  the  general  character  of  the  rock  than  by 
the  properties  of  the  mineral  itself.  They  can  be  easily  isolated 
from  the  rock  because  they  are  quite  heavy  and  are  not  attacked 
very  much  by  hydrofluoric  acid. 


Margarite  (9) 

Margarite  is  undoubtedly  much  more  widespread  than  it  has  been  shown 
to  be  up  to  the  present  time.  It  is  a  constant  associate  of  emery  and  is 
also  found  in  contact  rocks  rich  in  alumina.  It  is  distinguished  from  mica 
by  the  index  of  refraction  and  double  refraction,  and  from  colorless  chlorite 
by  the  former  property.  It  can  scarcely  be  distinguished  from  the  brittle 
micas  in  thin  section  except  by  the  very  large  optic  angle.  A  lower  hard- 
ness can  be  observed  on  the  larger  isolated  individuals. 

Olivine  Group  (9) 

Olivine,  or  peridote,  is  by  far  the  most  frequent  mineral  of  the 
olivine  group.  Forsterite  and  monticellite  are  rare  and  confined 
to  contact-metamorphosed  rocks.  Fayalite,  ferrous  silicate,,  in 

crystals  often  with  tridymite  and 
alone  in  compact,  dark  brown 
masses,  is  characteristic  in  certain 
rocks  and  is  probably  of  pneumato- 
lytic  origin.  Chemically,  the  mem- 
bers are  quite  similar,  but  their 
optical  properties  vary  greatly.  The 
indices  of  refraction  and  the  double 
refraction,  likewise  the  positive  optic 
angle,  increase  very  appreciably  with 

the  content  of  iron,  while  the  double  refraction  is  very  much 
lowered  by  the  content  of  calcium,  as  noted  in  monticellite. 
Members  rich  in  iron  fuse  more  easily  than  those  poor  in  iron. 


FIG.  281. 
Olivine  Tabular 
Parallel  (100). 


\ 


FIG.  282. 
Olivine  Tabular 
Parallel  (010). 


DESCRIPTIVE  SECTION 


273 


Olivine  with  an  average  content  of  ferrous  iron  of  from  11-13 
per  cent,  is  the  most  important  member  of  the  group.  Hyalo- 
siderite  is  much  richer  in  iron.  Olivine  is  characteristic  of  the 
most  basic  eruptive  rocks.  It  seldom  possesses  crystal  form  in 
the  granular  rocks  of  the  peridotite  series  except  when  numerous 
individuals  are  intergrown  poikilitically  in  pyroxene.  The 
same  is  true  in  gabbro,  norite,  and  trap.  On  the  other  hand,  it 
is  one  of  the  first  products  to  form  in  porphyric  rocks,  especially 
in  melaphyre  and  basalt,  and  less  often  in  andesite,  and  then  it  is 
well  bounded  crystallographically.  It  is  also  found  in  large 
amounts  in  certain  soda  rocks.  A  mineral,  which  appears 
quite  similar  and  is  widespread  in  contact  rocks,  has  a  much 


FIG.  283.— Olivine,  Section 
Parallel  (100). 


FIG.  284. — High 
Index  of  Mineral  in  Canada 


lower  content  of  iron  and  is  classified  as  forsterite.  In  general 
the  crystals  are  short  prismatic  parallel  to  the  vertical  axis  or 
tabular  parallel  to  the  macropinacoid,  Fig.  281.  They  are  less 
often  tabular  parallel  to  the  brachypinacoid,  Fig.  282,  and  that 
particularly  in  mixtures  rich  in  lime.  They  show  poorly  defined 
cleavage  cracks  parallel  to  the  direction  of  elongation  and  the 
plane  of  the  optic  axes  is  perpendicular  to  them,  Fig.  283.  Faces 
and  edges  are  generally  rounded  and  corroded,  and  indentations 
filled  with  the  solidified  magma  are  not  rare,  Fig.  284.  Basal 
sections  are  generally  eight-sided  and  show  two  unequal  cleav- 
ages perpendicular  to  each  other,  Fig.  285.  The  mineral  is  not 
often  prismatic,  except  when  it  is  rich  in  lime.  Then  the  prin- 
cipal zone  is  parallel  to  the  a  axis  and  is  positive.  Olivine  is 

18 


274 


PETROGRAPHIC  METHODS 


occasionally  found  with  a  skeletal  development  as  a  constituent 
of  the  ground  mass  in  rocks  poor  in  olivine  and  containing 
abundant  glass.  Then  it  shows  domatic  or  swallow-tail  forms 
and  various  thread-like  growths,  Fig.  196,  page  188. 

It  may  be  remarked  that  olivine  is  present  in  numerous  rocks 
of  the  basalt  series  in  large  sharp  angular  fragments  and  grains. 
These  can  be  referred  to  the  inclusions  of  olivine  fels  which  occur 
in  these  rocks.  The  entire  olivine  content  of  the  rock  may  be 
traced  back  to  crushing  of  these  inclusions.  In  this  form  it  is  not 
equivalent  to  the  other  constituents  of  a  basalt. 

Olivine  is  macroscopically  bottle  green  (hyalosiderite 
reddish-brown  to  golden  yellow)  and  has  a  distinctive  con- 

choidal  fracture.  Under  the  micro- 
scope it  is  almost  colorless,  and 
transparent,  but  when  heated  in 
air  it  always  becomes  reddish- 
brown.  The  ray  vibrating  parallel 
to  the  vertical  axis  is  then  less 
strongly  absorbed.  Hyalosiderite 
shows  this  property  when  fresh. 
Sometimes  twinning  is  noted  proba- 
bly parallel  to  {011}  and  this  can 
also  be  developed  in  the  form  of  laminations.  It  is  very  brittle 
and  frequently  shows  the  cataclastic  structure  under  the  influ- 
ence of  mountain-forming  processes. 

Its  characteristic  associate  is  chromite,  which  nearly  always 
occurs  as  small,  sharp  crystals  included  in  the  olivine.  Inclusions 
of  other  opaque  ores  and  apatite  are  also  found  in  olivine.  Like- 
wise, the  little  brown  tabular  crystals,  which  are  regularly 
arranged  and  are  so  characteristic  in  hypersthene,  occur  in  olivine 
of  certain  gabbros  so  abundantly  that  the  mineral  appears  black 
macroscopically  and  in  thin  section  it  is  scarcely  transparent. 
Glass  and  slag  inclusions  are  not  rare  in  basaltic  rocks,  and  liquid 
inclusions  are  found  in  peridotites  as  well  as  in  the  olivine  fels 
bombs  of  basalts. 

In  certain  gabbros  olivine  is  surrounded  by  a  wreath  of 
amphibole  arranged  radial  to  the  grain.  If  it  borders  on  feld- 
spar this  wreath  is  composed  of  common  hornblende,  but  other- 
wise it  is  tremolite.  All  of  the  olivine  may  finally  be  replaced 
by  a  confused  fibrous  aggregate  of  tremolite — pilite.  This  is 
frequently  considered  to  be  a  result  of  dynamometamorphism 


(100) 
FIG.  285. — Olivine,  Basal  Section. 


DESCRIPTIVE  SECTION 


275 


FIG.  286. 

Hyalosiderite  Border  around  Olivine,  Lim- 
burgite.      Limburg  on  Kaiserstuhl. 


and  is  genetically  entirely  different  from  serpentinization.  It 
may  be  noted  as  contradictory  to  this  hypothesis  that  tremolite 
is  the  most  frequent  by-product  in  the  formation  of  serpentine. 

In  basaltic  rocks  olivine  ap- 
pears to  be  generally  richer  in 
iron  and  often  shows  a  brilliant 
brown  border  of  hyalosiderite 
with  a  stronger  double  refrac- 
tion, Fig.  286.  Similar  borders 
seem  to  be  of  secondary  origin, 
the  olivine  having  been  altered 
into  a  yellow  to  reddish-brown 
substance  consisting  of  parallel 
fibers  with  very  high  indices  of 
refraction.  This  has  been  de- 
scribed as  goethite,  but  does 
not  correspond  to  it  in  all  of  its 
properties.  It  probably  is 
another  ferric  hydroxide. 

Alteration  into  scaly  aggregates  of  talc  or  granular  aggregates  of 
carbonate,  which  is  frequently  very  poor  in  magnesia,  is  noticed 
especially  in  melaphyres.  Now  and  then  biotite  also  results. 
By  far  the  most  frequent  alteration  of  olivine  is  into  serpentine, 
which  is  generally  accompanied  by  a  separa- 
tion of  iron  ores.  It  gives  the  rock  a  green 
color,  veined  or  streaked  with  various  shades 
of  red,  brown,  black,  etc. 

Two  kinds  of  serpentine  formations  are  dis- 
tinguished in  olivine.  One  begins  on  the  border 
of  the  crystals  and  in  the  cleavage  cracks  and 
forms  parallel  fibrous  aggregates  perpendicular 
to  them.  It  is  called  fibrous  serpentine  or 
chrysotile.  New  cracks  result  from  the  in- 
crease in  volume  accompanying  the  change 
and  these  are  filled  with  fine  fibrous  aggre- 
gates of  serpentine.  This  gives  rise  to  the 
typical  mesh  structure  shown  diagrammati- 
cally  in  Fig.  287,  and  as  it  appears  in  thin  section,  Fig.  218, 
page  199.  Checked  olivine  may  be  present  within  the  meshes 
or  this  may  be  entirely  changed  to  a  compact  aggregate  of 
serpentine. 


FIG.  287.— Olivine 
Beginning  Alteration 
to  Chrysotile,  Mesh 
Structure. 


276 


PETROGRAPHIC  METHODS 


In  other  rocks,  stubachite,  one  may  note  that  the  fresh  olivine 
is  crossed  by  numerous  lamellae  of  flaky  serpentine — anti- 
gorite.  These  are  mostly  orientated  parallel  to  {011}  and  form 
a  system  of  laths  cutting  each  other  at  an  angle  of  about  60°, 

Fig.  288.  The  angular  parts  of 
the  olivine  lying  between  them 
alter  into  irregular,  scaly  aggre- 
gates of  antigorite  beginning  at 
the  laths  and  progressing  out- 
ward. When  serpentinization 
is  complete,  the  intersecting 
lamellae  produce  the  so-called 
lattice  structure.  In  both  these 
cases  the  process  of  serpentini- 
zation cannot  be  looked  upon 
as  a  weathering  process;  see 
Allgemeine  Gesteinskunde,  page 
FlGA  288-T°u™e  ?nte'f  0™  with  152.  Sometimes  alteration  of 

Antigorite.     Lattice  Structure. 

olivine,     very     rich     in     iron, 

produces  a  flaky  aggregate  of  serpentine  orientated  in  a  simpler 
manner.  It  has  a  rust  brown  color  and  is  called  Iddingsite, 
see  under  serpentine. 

The  significance  of  fayalite  as  a  rock  constituent  has  not  been  investigated 
very  much,  but  it  is  known  principally  in  druses  and  in  mineral  veins. 
Possibly  some  of  the  formations  called  hyalosiderite  belong  to  it.  At  any 
rate  olivine  in  lamprophyric  rocks  is  often  very  rich  in  iron  on  the  border 
having  brownish  color  and  showing  increase  in  the  double  refraction. 

Titanium  olivine  is  quite  similar  to  it.  It  is  macroscopically  dark  brown 
and  yellow  to  orange  in  thin  section.  It  is  probably  monoclinic,  but  occurs 
in  grains  laminated  by  twinning  and  these  appear  similar  to  chondrodite. 
a  =  1.669,  /?-1.678,  r  =  1.702.  r-a  =  0.033,  2V  =  63°,  p<v,  a  orange, 
6  <=  c  light  yellow.  Sp.  gr.  =  3 . 25.  They  can  only  be  distinguished  by  the 
difference  in  extinction  in  sections  showing  twinning  lamination  and  the 
optical  orientation  of  such  a  section,  as  shown  in  Figs.  289  and  290. 

Forsterite  has  only  been  observed  in  contact-metamorphosed  limestones 
and  dolomites.  It  is  widely  disseminated  in  them,  but  is  generally  not 
macroscopically  visible.  When  the  carbonate  predominates  it  forms 
rounded,  colorless  crystals,  often  containing  quite  large  inclusions  of  iron 
spinel  and  calcite.  If  it  or  other  silicates  are  the  principal  constituents, 
it  forms  irregular  grains  with  the  properties  of  olivine.  Eozoon  results  by 
serpentinization  which  takes  place  in  the  ordinary  manner  and  is  quite 
widespread. 

Monticellite  is  analogous  to  forsterite  in  its  distribution,  but  is  much 
rarer.  The  lower  double  refraction  is  especially  noteworthy  and  distin- 


DESCRIPTIVE  SECTION 


277 


guishes  it  from  all  other  olivine  minerals.  It  forms  isolated,  colorless 
rounded  crystals  in  contact  limestones  and  these  often  show  incipient 
serpentinization. 

Confusion  of  titanium  olivine  with  chrome  chondrodite  has  already  been 
mentioned.  Forsterite,  colorless  olivine,  and  monticellite  often  appear 
quite  similar  to  diopside,  but  the  latter  shows  sharper  cleavage  cracks, 
larger  extinction,  and  has  the  optical  plane  orientated  parallel  to  the 
principal  zone.  They  can  be  distinguished  from  colorless  humite  only 
by  a  somewhat  stronger  dispersion  of  the  optic  axes  if  the  cleavage  does 


FIG.  289. — Chondrodite, 


FIG.  290.— Titanium  Olivine, 


Optical  Orientation  with  Respect  to  the  Twinning. 

not  appear.  If  the  cleavage  shows  distinctly  the  optical  plane  in  humite 
lies  parallel  to  it,  in  the  olivine  minerals  perpendicular.  In  other  cases  the 
olivine  is  distinguished  by  its  solubility  in  hydrochloric  acid  and  by  its 
ability  to  assume  a  brown  color,  which  increases  with  the  content  of  iron, 
when  it  is  roasted  in  an  oxidizing  flame.  This  latter  property  can  serve 
also  to  distinguish  olivine  from  forsterite  and  monticellite. 

Pyroxene  Group  (10) 

Orthorhombic  and  monoclinic  pyroxenes  frequently  occur  in 
parallel  or  lamellar  intergrowths.  The  triclinic  members  play 
no  r61e  among  rock-forming  minerals.  From  this  fact  the  ori- 
entation of  the  orthorhombic  pyroxenes  in  the  table  is  so  chosen 
that  the  acute  prism  angle  lies  to  the  front  and  the  transverse 
axis  is  the  shorter  diagonal,  although  this  is  not  the  usual  posi- 
tion. Prismatic  cleavage  is  nearly  always  distinct  and  if  other 
cleavages  are  present  the  one  parallel  to  the  front  pinacoid 
tends  to  be  the  most  perfect.  It  occurs  particularly  in  ortho- 
rhombic  pyroxenes  and  in  diallage.  Cleavage  plates  of  the 
former  parallel  to  this  face  are  parallel  to  the  optic  plane, 
while  those  of  the  latter  are  almost  perpendicular  to  an  optic 
axis  showing  quite  strong  dispersion.  A  large  extinction  angle 


278 


PETROGRAPHIC  METHODS 


(001) 


(001) 


FIG.  291. — Enstatite, 


FIG.  292.— Hypersthene, 


Section  Parallel  (100). 


(100) 


(010) 
(110) 


(nor 


I  (010) 


/(HO) 


Fia.^293. — Enstatite  and  Hypersthene, 


(100) 
FIG.  294.— Diallage, 


Cross  section. 


FIG.  295.— Diopside  (Diallage), 


(001) 


FIG.  296. — Aegirine, 


Section  Parallel  Plane  of  Symmetry. 


DESCRIPTIVE  SECTION  279 

characterizes  the  monoclinic  pyroxenes  with  the  exception  of 
those  rich  in  sodium.  They  are  further  distinguished  from  the 
orthorhombic  members  by  a  much  higher  double  refraction  and 
by  pleochroism,  which  is  particularly  typical  for  orthorhombic 
pyroxenes.  It  is  also  noteworthy  that,  unlike  the  amphibole 
group,  the  double  refraction  increases  with  the  content  of  sodium. 
The  pyroxenes  can  be  isolated  quite  easily  from  rocks  by  hydro- 
fluoric acid  because  they  are  very  slowly  attacked  by  it.  One 
of  the  most  important  means  for  determining  all  pyroxenes  is 
the  very  good  cleavage  parallel  to  the  prism  forming  an  angle 
of  approximately  90°.  This  distinguishes  them  from  the  am- 
phiboles  with  a  more  perfect  prismatic  cleavage  forming  an 
obtuse  angle. 

(a)  Orthorhombic  Pyroxenes 

Enstatite  and  bronzite  are  low  in  iron  and  may  show  various 
shades  of  brown  macroscopically.  Hypersthene  is  high  in  iron 
and  is  black.  They  form  large  irregular  individuals  in  the  granu- 
lar rocks  of  the  gabbro-peridotite  series  and  in  the  olivine  fels 
inclusions  in  basalt,  but  hypersthene  is  not  so  common  as  the 
others.  They  show  very  perfect  cleavage  parallel  to  the  macro- 
pinacoid,  but  always  in  displaced  cracks,  and  generally  have  a 
metallic  shimmer  caused  by  parallel  orientated  inclusions  of 
brown  tabular  crystals  and  laths  (ilmenite?).  This  passes  over 
into  golden  yellow  upon  beginning  alteration  into  serpentine — 
bastite.  Macroscopically,  they  are  generally  various  shades  of 
brown,  but  in  thin  section  only  those  members  rich  in  iron  are 
colored,  and  then  they  show  characteristic  pleochroism,  but  no 
distinct  difference  in  the  intensity  of  the  colors.  Very  frequently 
an  indistinct  twinning  lamination  occurs,  which  passes  over  into 
the  formation  of  fine  fibers.  Likewise  lamellar  intergrowths 
with  diallage  can  be  distinctly  discerned  even  in  sections  showing 
parallel  extinction,  because  of  the  different  optical  orientation 
of  orthorhombic  and  monoclinic  pyroxenes. 

Well  developed  crystals,  invisible  macroscopically,  are  widely 
disseminated  in  porphyrites  and  andesites,  and  in  these  hyper- 
sthene generally  predominates.  The  crystals  are  short  prismatic 
with  rounded  pyramidal  ends  and  in  the  vertical  zone  they  are 
principally  bounded  by  the  end  faces,  Fig.  293.  They  are  often 
rich  in  inclusions  of  light  colored  glass  or  slag.  In  these  the 
cleavage  is  only  prismatic  and  pleochroism  becomes  a  char- 


280  PETROGRAPHIC  METHODS 

acteristic  property.  Parallel  intergrowths  with  monoclinic  py- 
roxenes are  found  in  which  the  latter  form  a  shell  around  a 
center  of  the  orthorhombic  variety.  Glass  inclusions  and  the 
rounding  of  the  edges  are  frequent,  but  orientated  inclusions 
and  fibrous  structure  are  lacking.  A  second  generation  of  hy- 
persthene  in  microlitic  development  is  observed  now  and  then 
in  the  ground  mass  of  andesitic  rocks. 

Alteration  is  comparatively  frequent,  especially  that  into  ser- 
pentine which  affects  the  orthorhombic  pyroxenes,  particularlv 
when  they  occur  as  constituents  of  serpentinized  olivine  rocks. 
An  analogous  alteration  is  also  observed  in  the  crystals  in  por- 
phyries. A  parallel,  scaly  aggregate  of  antigorite  is  formed 
progressing  from  the  transverse  cracks  outward,  and  the  faces, 
parallel  to  which  antigorite  possesses  perfect  cleavage,  lie  parallel 
to  the  macropinacoid  of  the  pyroxene.  Cleavage  plates  of  the 
pseudomorphs — bastite — thus  formed,  are  perpendicular  to  the 
acute  bisectrix  and  the  optic  angle  varies  as  is  characteristic  for 
serpentine.  The  names  diaclasite,  protobastite,  etc.,  refer  to 
masses  of  serpentine  with  pyroxene  which  is  still  fresh.  Horn- 
blende is  formed  now  and  then  at  the  expense  of  orthorhombic 
pyroxene  in  granular  rocks. 

Aside  from  the  parallel  extinction,  orthorhombic  pyroxenes  are 
distinguished  from  the  monoclinic  members  by  a  much  lower 
double  refraction  and  from  aegirine  by  the  positive  character 
of  the  principal  zone.  They  are  distinguished  from  andalusite, 
olivine,  etc.,  by  a  more  distinct  development  of  the  cleavage  and 
by  the  character  of  the  principal  zone,  which  is  positive  in  ortho- 
rhombic  pyroxenes,  negative  in  andalusite,  and  variable  in 
olivine. 

(b)  Monoclinic  Pyroxenes 

Monoclinic  pyroxenes,  free  from  sesquioxides,  are  called  diop- 
sfde  normally.  It  is  one  of  the  most  characteristic  minerals  in 
contact  rocks  rich  in  lime  and  occurs  in  short  prisms  or  grains, 
which  are  macroscopically  light  grayish-green  or  less  often  sap- 
green.  It  is  found  in  eclogites,  as  omphazite  containing  a  little 
aluminium  and  in  certain  peridotites  as  chrome  diopside  con- 
taining chromium.  It  is  sometimes  blackish-green  in  heden- 
bergite  rich  in  iron,  which  occurs  only  in  the  silicate  aggregates 
known  as  skarn.  The  mineral  is  generally  colorless  in  thin  sec- 
tion, but  may  show  a  pale  greenish  tinge.  The  deeper  colored 


DESCRIPTIVE  SECTION 


281 


varieties  are  also  green  under  the  microscope  and  hedenbergite 
is  deep  green,  but  shows  no  pleochroism.  The  presence  of  pleo- 
chroism,  except  in  the  case  of  fassaite,  seems  to  be  connected 
with  the  presence  of  alkali  silicates. 

Pyroxene  similar  to  diopside  is  found  in  acid  eruptive  rocks 
from  granite  to  diorite — malacolite — and  especially  in  the  basic 
differentiation  products  from  minette  to  kersantite.  In  certain 
varieties  of  trap,  a  diopside  occurs  along  with  common  augite, 
described  below,  but  it  is  distinguished  by  a  high  content  of 
magnesia — salite.  A  very  small  optic  angle  is  observed  in  these 
occurrences. 

In  general  they  form  short  prismatic  crystals,  Fig.  297,  or 
tabular  crystals  parallel  to  the  orthopinacoid,  Fig.  298.  The 
cleavage  shows  quite  distinctly  in  thin  section.  They  are  more 
or  less  crystallographically  bounded  even  where  the  common 
augite  forms  the  interstitial  material.  The  form  of  the  pyrox- 


\ 


FIG.  297.— Augite, 
Short  Prismatic. 


FIG.  298. — Augite,  Tabular 
Parallel  (100). 


enes  in  the  extrusive  rocks  of  the  acid  and  intermediate  series 
is  quite  similar  to  the  above.  Pyroxenes  are  found  as  phenocrysts 
especially  in  andesites.  Diallage  includes  those  varieties  which 
are  distinguished  from  diopside  by  a  low  content  of  alumina.  It 
shows  basal  parting  macroscopically  as  well  as  parting  parallel 
to  the  orthopinacoid,  and  also  twinning  laminations  parallel  to 
it.  It  has  a  metallic  chatoyancy  and  the  individuals,  which  occur 
particularly  in  gabbro  and  peridotite,  are  quite  distinctly  visible 
macroscopically.  Under  the  microscope  it  is  often  somewhat 
pleochroic  and  aside  from  its  occasional  fibrous  properties,  is 
marked  by  the  regular  arrangement  of  small,  brown,  tabular 
inclusions,  mentioned  under  bronzite.  It  is  not  to  be  considered 
as  an  independent  mineral  species. 

Fassaite,  which  is  chemically  quite  closely  related  to  diallage, 
plays  a  peculiar  role.     It  is  found  principally  as  a  constituent  of 


282  PETROGRAPHIC  METHODS 

contact  rocks,  but  is  also  quite  important  in  monzonites.  It 
corresponds  to  aegirine,  described  below,  in  its  optical  properties, 
especially  in  the  pleochroism  and  strong  inclined  dispersion,  but 
differs  greatly  in  chemical  constitution.  The  sharp  pyramidal 
habit  exhibited  by  the  crystals  in  an  extraordinary  manner  is 
distinctive  for  it. 

There  is  a  series  of  gradual  transition  members  from  diopside 
to  common  or  basaltic  augite,  the  latter  being  characterized  by  a 
higher  content  of  alumina.  The  mineral  is  always  black  macro- 
scopically,  but  under  the  microscope  it  generally  has  only  a  very 
pale,  brownish  tint  and  is  not  normally  pleochroic.  Common 


FIG.  299.— Augite  as  Interstitial  Material.  FIG.  300.— Simple  Twinning 

Diabase.     Arran,  Scotland.  Lamellae  in  Augite. 

augite  is  disseminated  first  of  all  in  the  basic  eruptive  rocks,  and 
in  porphyric  rocks  it  often  occurs  in  two  generations  and  the 
first  of  these  often  forms  phenocrysts  of  considerable  size.  The 
occurrence  of  diabase  augite  as  interstitial  material,  Fig.  299, 
is  especially  distinctive.  The  formation  of  twins  is  quite  common 
in  common  augite,  especially  in  the  form  of  isolated  lamellae. 
Juxtaposition  and  penetration  twins  are  also  found  and  the 
hourglass  structure,  which  is  frequently  observed,  is  also  looked 
upon  as  complicated  twinning.  The  twinning  lamellae  are  ar- 
ranged parallel  to  {lOO}  and,  therefore,  in  a  pyramidal  section 
they  lie  oblique  to  the  cleavage  parallel  to  the  prism  faces, 
Fig.  300.  Many  kinds  of  forms  of  growth,  crystal  skeletons,  etc., 
are  found  in  glassy  eruptive  rocks. 

A  very  large  extinction  angle  is  a  characteristic  of  this  augite 
and  it  shows  particularly  in  comparison  with  the  hornblende, 


DESCRIPTIVE  SECTION  283 

which  occurs  as  a  border  around  such  augite.  If  the  small 
content  of  titanium,  which  is  usually  present,  increases  a  dis- 
tinct violet  color  generally  appears  in  thin  section.  Such  titan- 
ium augites  are  somewhat  pleochroic.  The  ray  vibrating  parallel 
to  b  is  colored  deep  violet.  Sections  with  the  most  oblique 
extinction  show  the  least  pleochroism.  Locally,  reddish  to 
brownish-red  colors  are  observed,  and  these  can  be  referred  to  a 
content  of  manganese.  However,  some  occurrences  of  greenish 
or  brownish  augites  assume  that  color  when  roasted  in  air. 
The  color  of  augite  is  rarely  yellow. 

Alteration  in  diopside  and  augite  is  very  widespread.  In  the 
first  case  the  decomposition  gives  rise  to  fibrous  scaly  aggregates, 
which  are  generally  considered  to  be  serpentine,  but  which 
scarcely  belong  to  it.  Augites  rich  in.  aluminium  are  generally 
changed  to  chlorite.  Low  doubly  refracting  aggregates  of  it 
are  congregated  on  the  borders  and  in  cleavage  cracks  of  the 
individuals,  and  give  rise  to  the  formation  of  the  greenstone 
facies  of  augite  rocks.  They  are  usually  penetrated  by  grains  of 
epidote  and  calcite,  and  frequently  also  by  small  needles  of 
hornblende.  They  include  small  individuals  of  titanite,  anatase, 
and  opaque  ores.  It  is  noteworthy  that  this  alteration  is  every- 
where accompanied  by  the  formation  of  pyrite.  Sometimes  an 
aggregate  of  quartz  with  carbonates  and  rust  occurs  instead  of 
the  green  minerals  in  a  rock  that  is  entirely  decomposed  and 
changed  to  wache.  This  kind  of  alteration  appears  in  its  entire 
distribution  to  be  a  direct  antithesis  to  the  first. 

Uralite  is  an  especially  interesting  pseudomorph.  It  is  gen- 
erally green,  but  sometimes  bluish  hornblende  in  the  form  of 
augite.  It  was  generally  considered  as  a  product  of  paramor- 
phism,  but  it  is  a  typical  product  of  pneumatolytic  hydatogenic 
activity  in  which  anhydrous  augite  passes  over  into  hornblende 
containing  an  hydroxyl.  This  is  frequently  accompanied  by  an 
intensive  alteration  of  its  entire  composition.  In  porphyric 
rocks — uralite  porphyrite — the  form  of  the  augite  is  perfectly 
retained  and  the  formation  of  fibrous  hornblende  attacks  the 
whole  crystal  without  change  of  form.  In  granular  rocks  on 
the  other  hand,  e.g.,  in  uralite  gabbro,  the  newly  developed 
hornblende  grows  like  brushes  spreading  out  over  the  crystals 
so  that  uralitization,  which  frequently  goes  hand  in  hand  with 
saussuritization,  entirely  obscures  the  structure  of  the  rock. 

Although  the   alteration  first  mentioned    is    interpreted    as 


284  PETROGRAPHIC  METHODS 

weathering,  the  latter  type  is  a  typical  result  of  dynamometamor- 
phism,  see  Allgemeine  Gesteinskunde  page  151.  Recrystallization 
from  magmatic  waters  may  be  the  cause,  and  it  cannot  be  doubted 
in  the  alteration  to  seladonite  where  an  augite  rock  free  from 
potassium  is  transformed  into  a  green  silicate  rich  in  potassium. 
This  occurs  in  a  very  characteristic  manner  in  the  melaphyres 
of  the  Fassatal. 

Augite  is  itself  not  rare  as  a  secondary  product,  and  forms 
the  principal  constituent  of  resorbed  residues  of  hornblende  or 
mica  in  extrusive  rocks.  It  is  a  typical  product  of  crystallization 
from  a  magma  after  its  extrusion  upon  the  surface  and  the  dark 
minerals  formed  during  the  intratelluric  period  of  the  magma  are 
aggregates  of  augite  grains  mixed  with  opaque  ores. 

Diopside  is  often  uncommonly  plastic  when  subjected  to  oro- 
genic  pressure,  and  its  prismatic  individuals  are  frequently 
greatly  bent.  Augite  and  diallage  are  more  inclined  to  the  de- 
velopment of  cataclastic  structure,  which  strangely  enough  occurs 
just  as  perfectly  developed  in  young  eruptive  rocks,  which  have 
not  suffered  any  kind  of  deformation,  and  is  therefore  a  protoclase. 

The  members  of  the  series  containing  sodium,  leading  from 
augite  or  diopside  to  aegirine,  are  confined  to  the  soda  rocks,  and 
are  characterized  as  a  whole  by  distinct  pleochroism,  which  in  the 
normal  occurrences  alternates  between  yellow  and  green  shades, 
and  is  deeper  green  the  richer  they  are  in  sodium.  If  they 
contain  titanium,  as  is  frequently  the  case  in  basalts,  they  show 
a  violet  color  principally  in  the  direction  of  B.  The  titanium 
augites  containing  sodium  are  strongly  pleochroic  and  the  hour- 
glass structure,  mentioned  on  page  188,  is  especially  conspicuous 
in  them.  The  members  containing  sodium,  as  a  whole,  fre- 
quently show  fine  zonal  development  in  which  the  very  narrow 
zones  alternate  with  each  other  in  an  extremely  regular  manner. 
Sometimes  a  more  or  less  intensely  corroded  and  irregular  grain 
is  surrounded  by  shells,  which  are  crystallographically  well 
defined.  In  the  soda  rocks  it  may  be  observed  that  in  the 
poorly  bounded  pyroxenes,  which  are  frequently  tabular  parallel 
to  the  front  pinacoid,  the  core  is  almost  colorless,  non-pleochroic, 
and  is  manifestly  closely  related  to  diopside.  Toward  the 
border  it  passes  into  aegirine  augite  and  finally  the  outer  zone 
is  aegirine.  Intergrowths  with  hornblende  are  not  at  all  rare 
but  then,  unlike  the  augite,  hornblende  containing  sodium  is  in 
the  center  and  the  pyroxene  grows  around  it. 


DESCRIPTIVE  SECTION 


285 


With  reference  to  extinction  the  monoclinic  pyroxenes  differ 
from  each  other  in  a  distinctive  manner.  From  diopside  to 
common  augite,  and  from  this  to  aegirine,  the  angle  between 
the  positive  bisectrix  and  the  vertical  axis  becomes  constantly 
greater,  Fig.  301. 

The  soda  pyroxenes  show  crystal  form  particularly  in  the 
porphyric  rocks  in  which  they  occur  in  two  generations.  The 
large  phenocrysts  often  show  tabular  development  parallel  to 
{100}  while  the  second  generation  is 
always  richer  in  sodium  and  is  fine 
acicular  showing  a  radial  development. 
In  other  cases,  the  latter  type  forms 
short  irregular  specks,  which  are  often 
so  intergrown  with  small  nepheline  crys- 
tals that  they  are  scarcely  transparent. 
The  soda  pyroxenes,  especially  acmite, 
which  is  generally  spear-shaped,  show 
extremely  irregular  forms,  but  not  to 
the  same  degree  as  the  corresponding 
amphiboles.  Manifold  crystal  skeltons 
are  found  in  glassy  rocks.  It  cannot  be 
positively  determined  whether  the  forms 
of  growth  illustrated  on  page  188  are  aegirine  or  hornblende. 

Sodium  augite  and  the  titanium  augites,  belonging  to  it,  are 
characterized  by  a  strong  inclined  dispersion,  as  a  result  of  which 
sections  parallel  to  the  plane  of  symmetry  give  an  extremely 
incomplete  extinction,  and  in  sections  perpendicular  to  the  optic 
axis  very  brilliant  anomalous  interference  colors  are  seen.  The 
difference  in  this  respect  from  diopside  is  notable.  It  has  a  very 
small  dispersion  and,  therefore,  its  extinction  in  the  plane  of 
symmetry  is  very  complete.  Alteration  of  the  soda  pyroxenes 
is  rare.  Sometimes  aggregations  of  opaque  ores,  which  are 
difficultly  determinable,  occur  in  place  of  them  in  entirely 
decomposed  keratophyres. 

Although  in  general  a  regular  relationship  between  the  optical 
properties  and  the  chemical  composition  of  monoclinic  pyroxenes 
cannot  be  denied,  yet  investigations  to  determine  the  chemical 
composition  merely  from  the  optical  properties  lead  to  rather 
negative  results.  Minerals  of  the  aegirine- augite  series,  which 
are  designated  as  fedorowite  or  violaite,  are  in  a  way  not  to  be 
considered  as  an  independent  species,  but  chemical  analysis 


FIG.  301. — Extinction    of 
Monoclinic  Pyroxenes. 


286 


PETROGRAPHIC  METHODS 


shows  great  deviations  from  what  would  be  expected  from  the 
optical  properties.  The  pyroxene  group  is  built  up  in  too  com- 
plicated a  manner  in  contradistinction  to  the  plagioclase  group, 
in  which  such  conclusions  appear  to  be  well  justified. 

Colorless  pyroxenes  are  most  frequently  confused  with  olivine 
or  epidote,  but  are  distinguished  from  both  by  the  position  of  the 
optical  plane  with  respect  to  the  zone  of  cleavage.  The  colored 
members  are  sometimes  very  similar  to  common  hornblende  and, 
if  the  depth  of  color  is  the  same  in  each,  the  absorption  of  the 
latter  in  the  principal  zone  is  stronger.  The  extinction  is  less 
with  the  exception  of  aegirine,  which  can  be  recognized  however 
by  its  sap  green  color,  higher  double  refraction,  and  the  negative 
character  of  the  principal  zone.  Under  certain  circumstances, 
the  cleavage  angle  of  pyroxene  approximates  that  of  hornblende 
in  oblique  sections.  If  these  sections  have  parallel  extinction 
the  optic  plane  lies  in  the  bisector  of  the  acute  cleavage  angle 
in  pyroxene  and,  in  general,  in  the  bisector  of  the  obtuse  cleavage 
angle  in  hornblende. 

Soda  pyroxenes  poor  in  iron  or  free  from  iron  are  comparatively  rare  as 
rock  constituents.  They  are  macroscopically  white  to  green,  but  under  the 
microscope  generally  colorless.  Jadeite  (NaAlSi2O6)  belongs  especially  to 
this  group.  It  occurs  only  in  fine  columnar  masses  with  properties  like 

saussurite,  in  the  serpentine  from  Burmah, 
Turkestan,  etc.,  but  is  widely  known  from  the 
stone  age  when  it  was  worked  up.  It  is  easily 
distinguished  from  the  similar  nephrite  by  its 
low  fusibility.  Further,  spodumene  must  be 
mentioned.  It  occurs  as  a  constituent  of 
pegmatite  in  clear  beautifully  colored  crystals, 
emerald  green  hiddenite  and  rose-red  kuntzite, 
and  as  a  constituent  of  other  rocks,  of  granites 
and  injected  schists,  in  poorly  developed  gray- 
ish-green needles,  which  are  purely  accessory. 

Lawsonite  (10) 


-#  (ooi) 


FIG.  302. — Lawsonite,  Section 
Parallel  (010). 


Lawsonite  has  been  found  as  a  constituent 
of  certain  eclogites.  It  forms  large  lobated 
particles  in  them  without  any  characteristic 
microstructure.  The  mineral  is  also  suspected 
in  certain  occurrences  of  saussurite,  but  the 

determination  does  not  seem  to  be  reliable  because  of  the  lack  of 
characteristic  properties  of  the  mineral  and  its  poor  development,  which 
all  the  minerals  in  saussurite  show.  In  such  aggregates  confusion  with 
hornblende  or  prehnite  is  natural,  but  it  is  distinguished  from  the  other 
minerals  of  the  aggregate  by  much  stronger  double  refraction.  Lawsonite 


DESCRIPTIVE  SECTION 


287 


is  distinguished  from  hornblende  by  its  constant  parallel  extinction  and 
the  direction  of  the  cleavage  cracks  in  macropinacoidal  sections  and  prehnite 
is  distinguished  by  the  characteristic  rosette  arrangement  in  its  cross  sec- 
tions. Stronger  double  refraction  and  positive  character  of  the  principal 
zone  distinguish  it  from  andalusite  and  the  position  of  the  optical  plane 
parallel  to  the  cleavage  from  wollastonite,  while  the  distribution  distin- 
guishes it  from  both. 

Amphibole  Group  (11) 

The  minerals  jof  the  amphibole  group  have  a  very  perfect 
cleavage  parallel  to  the  prism  making  an  angle  of  about  124°. 
They  always  show  a  prismatic  development,  Fig.  303, 
but  may  be  short,  thick  forms  like  the  crystals  of 
pargasite  or  basaltic  hornblende,  or  the  grains  of 
common  hornblende.  They  may  be  long,  needle-like 
and  fibrous,  as  is  often  the  case  in  the  light  green 
hornblendes  of  green  schists  and  the  secondary  aggre- 
gates in  diabase.  The  crystal  form  is  nearly  always 
observed  in  the  principal  zone  and  consists  of  the 
prism  and  clinopinacoid.  The  amphiboles  have  a  FIG  303 
tendency  to  show  a  defective  development  on  the  Hornblende, 
ends,  when  they  occur  as  phenocrysts  in  porphyries. 
Compare  with  aegirine  for  the  occurrence  of  very 
delicate  crystal  skeletons  in  certain  pitchstones.  Zonal  structure 
and  parallel  growths  of  various  members  of  .the  series  are  ob- 
served, although  they  are  not  numbered  among  the  character- 


FIG.  304. 
Actinolite, 


FIG.  305. 
Basaltic  Hornblende, 


FIG.  306. 
Glaucophane, 


Section  Parallel  Plane  of  Symmetry. 


288 


PETROGRAPHIC  METHODS 


istic  features  as  is  the  case  with  the  soda  pyroxenes.     See  under 

pyroxenes  for  parallel  growth  with  them. 

Unlike  the  pyroxenes,  with  which  they  can  be  confused  in 

color,  index  and  double  refraction,  the  amphiboles  have  a  very 

small  extinction  angle  of 
from  0°  to  25°  with  the 
exception  of  the  rare  group 
of  cataphorites.  This  is 
shown  in  Fig.  308  in  which 
the  relationship  of  c:  c  is 
diagrammed  for  the  most 
important  members.  This 

FIG.  307.— Cross  Section  of  Hornblende.  angle     has     a     Very     small 

value   in    the    amphiboles 

free  from  alkali,  but  in  soda  hornblende  the  angle  c :  a  is  small. 
The  minerals  of  the  amphibole  group  are  generally  deep  colored 
and  show  various  colors  even  in  thin  section.  With  the  ex- 
ception mentioned  above,  the  ray  vibrating  in  the  principal  zone 
is  most  strongly  absorbed.  The  plane  of  the  optic  axes  is  nearly 
always  parallel  to  the  plane  of  symmetry.  Only  in  the  case  of 
blue  hornblendes  rich  in  sodium  are 
these  two  planes  perpendicular.  The 
chemical  composition  of  the  minerals 
belonging  in  this  group  is  quite  varia- 
ble and  is  in  general  analogous  to  that 
of  the  pyroxenes,  but  differs  from 
them  in  the  content  of  the  hydroxyl, 
which  is  never  absent.  In  conse- 
quence of  this  fact,  the  deep  seated 
rocks  are  the  ones  which  contain  the 
hornblende,  while  augite  takes  its 
place  in  the  extrusive  rocks.  It  is 
further  observed  that  hornblende  is 
changed  to  augite  by  the  action  of 
heat,  magmatic  resorption,  while  hydro- 
chemical  processes  have  the  opposite 
effect  and  form  uralite.  The  specific 
gravity  and  indices  of  refraction  are  lower  in  hornblende  than  in 
the  pyroxenes,  which  have  an  analogous  composition,  but  the 
amphiboles  are  more  strongly  magnetic. 

Pleochroic  halos  with  brown  colors  are  common.     They  are 


FIG.  308. — Extinction  on  Mono- 
clinic  Amphiboles. 


DESCRIPTIVE  SECTION 


289 


also  observed  in  uralite  but  not  a  trace  of  them  can  be  found  in 
the  original  pyroxene.  The  index  and  double  refraction  are 
higher  in  the  halos. 

The  amphiboles  are  not  difficultly  fusible  and  those  rich  in 
sodium  even  belong  to  the  most  easily  fusible  minerals.  Green 
hornblende  turns  brown  upon  roasting  gently,  and  it  assumes  the 
absorption  and  optical  orientation  of  basaltic  hornblende. 

Anthophyllite  and  gedrite,  which  are  considered  orthorhombic 
because  of  the  parallel  extinction,  are  found  sporadically  in 
amphibolites  and  are  predominant  in  anthophyllite  schist. 
They  are  also  found  in  certain  serpentines.  They  form  flaky, 
columnar  or  tufted  aggregates,  and  less  often  small  acicular 
crystals.  They  possess  a  brownish  color.  Alteration  into  talc 
and  serpentine  is  known. 

Tremolite  usually  appears  in  fine  radiating  needles  with  white 
or  pale  greenish  color.  These  occur  in  granular  limestone  and 
dolomite,  or  in  serpentine.  In  the  latter  rock  there  are  all 
possible  transitions  from  trem- 
olite  to  actinolite,  which  is 
always  distinctly  green  macro- 
scopically,  and  in  thin  section 
it  is  generally  somewhat 
colored,  but  is  scarcely  pleo- 
chroic.  Both  types  are  some- 
times found  in  larger  prisms. 
The  occurrence  in  serpentine, 
which  is  uncommonly  wide- 
spread, especially  in  the  micro- 
scopical development,  must 
be  looked  upon  as  a  secondary 
by-product  in  the  formation 
of  serpentine.  Sometimes  the 
transition  from  olivine  direct  to  such  aggregates  of  amphibole 
with  spherulitic  structure  or  that  similar  to  snow  crystals  is 
observed — pilite.  An  analogous  development  also  occurs  in 
nephrite,  which  is  usually  light  green,  macroscopically,  and 
consists  of  a  dense  mat  of  actinolite  needles.  It  represents  the 
toughest  of  all  rocks.  Its  occurrence  is  also  connected  with 
serpentine.  Poorly  developed  individuals  of  tremolite  and 
actinolite  are  often  bent  by  mechanical  stresses  and  are  crushed 
into  isolated  columns,  which  show  a  parquetry  appearance  in 

19 


FIG.  309. — Distorted  Actinolite  in  Serpentine, 
Rifflkees.     Stubachtal,  Salzburg. 


290  PETROGRAPHIC  METHODS 

cross  section,  Fig.  309.  It  is  most  similar  to  the  normal  occur- 
rence of  prehnite  and  is  distinguished  by  the  obtuse  angle  of  the 
fragments.  These  minerals  also  form  fibrous  individuals  and 
pass  over  into  asbestos. 

Macroscopic  varieties,  with  a  sap  green  color,  which  are  often 
extremely  fine  fibrous  and  generally  contain  some  alumina  and 
occur  as  pseudomorphs  after  diallage,  are  called  smaragdite. 
In  thin  section  it  is  colorless  to  light  green  and  often  shows  the 
structure  and  inclusions  of  diallage  quite  distinctly. 

Among  the  amphiboles  free  from  alumina,  griinerite  (Fe- 
Mg)SiO3  is  remarkable  because  of  its  similarity  to  epidote. 
n  =  l.7.  f-  a  =  0.03-0. 056.  tt  =  B  colorless,  c  light  yellow. 
Sp.  gr.=3.3.  c:c=15°.  2E  =  95°.  Optically  positive.  Usu- 
ally shows  twinning  lamination  parallel  to  the  orthopinacoid. 
Brownish  cummingtonite,  (MgFe)Si03,  may  also  be  mentioned. 
a  =  B  light  yellow,  t  light  brownish,  c :  c  =  15°  and  7-  -  a  =  0 . 022. 
The  petrographical  importance  of  these  is  but  little  known. 

Edenite,  an  amphibole  poor  in  iron  and  rich  in  alumina,  is 
macroscopically  light  green,  but  generally  colorless  in  thin  section. 
It  cannot  be  distinguished  from  those  already  described.  It 
is  found  in  isolated,  rounded  and  greatly  corroded  crystals  with 
a  short  prismatic  development  in  granular  limestones  and  lime- 
silicate  fels  together  with  pargasite,  which  is  richer  in  iron  and  is 
macroscopically  darker,  being  green,  or  brown  to  black.  It 
forms  the  transition  to  common  green  hornblende  rich  in  alumina 
and  ferrous  iron.  The  latter  appears  blackish-green  macro- 
scopically. It  is  found  in  compact,  rarely  elongated,  prismatic 
grains  in  the  acid  and  intermediate  rocks  of  the  normal  series, 
and  shows  well  developed  crystals  in  the  porphyric  forms  of  these. 
Very  strong  absorption  is  always  distinctly  observed,  and  iso- 
lated twinning  lamellae  parallel  to  the  orthopinacoid  are  not 
rare.  In  contact  rocks  the  cross  sections  of  hornblende  often 
show  a  bluish-green  color  and  transitions  to  carinthine,  which 
is  deep  bluish-green  in  thin  section,  are  observed  especially  in 
the  sheaf-like  aggregates  of  the  Garbenschiefer.  Hornblende 
showing  a  similar  development  is  very  widespread  in  amphi- 
bolites  in  which  it  sometimes  forms  short  shreds  and  sometimes 
parallel  columnar  aggregates  with  frayed  ends — sedgy  horn- 
blende. The  latter  form  of  development  is  very  similar  to  the 
uralite  pseudomorphs  after  augite  found  in  numerous  basic 
eruptive  rocks.  In  such  rocks  the  mineral  is  often  very  light 


DESCRIPTIVE  SECTION  291 

.colored  and  is  developed  in  long  needles.  It  is  then  very  difficult 
to  distinguish  from  similar  occurrences  of  actinolite  without  mak- 
ing a  thorough  chemical  investigation.  The  stronger  absorption 
of  the  ray  vibrating  in  the  direction  of  elongation  is  nearly  always 
distinctly  observable  in  common  hornblende.  Green  hornblende 
with  such  a  development  is,  along  with  chlorite,  the  most  ^fre- 
quent green  pigment  of  schistose  rocks. 

A  brownish-green  variety  of  hornblende  occurs  in  place  of  the 
green  in  porphyrites.  The  crystals  are  short  prismatic  and  the 
ray  vibrating  parallel  to  b  is  very  distinctly  brownish.  This 
passes  over  into  pure  brown  varieties  rich  in  ferric  iron,  which  are 
lighter  or  deeper  colored  corresponding  to  the  content  of  iron. 
The  lightest  colored  varieties  of  common,  brown  hornblende  are 
macroscopically  brownish-black,  and  they  replace  the  green 
entirely  in  deep  seated  basic  rocks,  e.g.,  in  gabbro.  When  horn- 
blende is  primary  in  such  rocks  it  belongs  to  the  brown  variety. 
Secondary  processes  analogous  to  the  formation  of  uralite  alter 
the  brown,  primary  hornblende  into  secondary  green,  sedgy  horn- 
blende. Tonalite  and  certain  granulites  are  the  principal  acid 
rocks  which  contain  brown  hornblende.  It  is  also  found  as 
phenocrysts  in  numerous  porphyric  rocks  and  is  especially  char- 
acteristic for  certain  lamprophyres  in  which  small  needles  of 
brown  hornblende  are  the  principal  constituent  of  the  ground 
mass.  They  are  always  replaced  by  augite  in  the  ground  mass  of 
extrusive  rocks. 

If  titanium  dioxide  also  occurs  in  varieties  rich  in  iron,  the 
mineral  is  macroscopically  jet  black  and  has  a  pitchy  luster. 
The  cleavage  faces  are  exceptionally  brilliant  and  in  thin  section 
the  absorption  in  the  principal  zone  is  more  complete,  while  the 
extinction  diminishes  to  0°.  This  series  is  designated  as  basaltic 
hornblende  and  contains  up  to  5  per  cent.  Ti02.  It  is  widespread 
in  numerous  extrusive  rocks  in  which  the  crystals  are  long  or 
short  prismatic,  and  are  well  developed.  They  are  frequently 
zonal  with  a  light  center  and  darker  borders.  They  show  twin- 
ning parallel  to  the  orthopinacoid  most  frequently  and  this  some- 
times appears  as  isolated  lamellae  or  as  two  equal  parts  of  twins. 
The  indices  and  double  refraction  are  considerably  higher  than  that 
of  the  other  amphiboles.  On  the  border  it  often  shows  a  zone 
with  stronger  absorption.  It  has  resulted  from  the  action  of  the 
magma  on  the  mineral  after  it  was  formed — magmatic  border. 
This  leads  to  magmatic  resorption  in  which  pseudomorphs  of 


292  PETROGRAPHIC  METHODS 

augite  and  magnetite  after  hornblende  are  formed.  Finally 
even  the  form  is  destroyed.  Thus,  in  the  tuffs,  large  crystals  of 
basaltic  hornblende  are  present  but  in  the  extrusive  rocks 
specks  very  rich  in  iron  are  found  as  a  residue  from  them.  The 
varieties  richest  in  titanium,  6-7  per  cent.  TiO2,  are  called  Kar- 
sutite. 

Amphiboles  containing  aluminium  and  ferric  oxide  suffer  the 
same  alterations  as  common  augite,  but  not  to  the  same  extent. 
A  distinction  is  made  between  chloritization,  which  is  always 
accompanied  by  the  formation  of  epidote,  carbonates,  and  titan- 
ium minerals,  and  the  alteration  into  quartz,  carbonates,  and 
rust.  Amphiboles,  like  the  pyroxenes,  are  resistive  to  acids.  All 
green  members  assume  a  brown  color  and  stronger  absorption 
when  heated  in  air.  Those  members  richer  in  iron  show  the  op- 
tical properties  of  basaltic  hornblende  after  roasting. 

There  are  many  members  of  the  amphibole  group  that  are  rich 
in  sodium.  Two  groups  may  be  distinguished.  The  one  is  poor 
in  ferric  iron  and  rich  in  alumina  and  occurs  chiefly  in  contact 
rocks,  especially  in  eclogites.  The  other  is  rich  in  ferric  iron  and 
is  distributed  principally  in  the  rocks  of  the  soda  series. 

There  are  transition  members  from  common,  green  hornblende 
to  carinthine,  which  is  macroscopically  bluish-black,  and  in  thin 
section  is  deeply  colored.  The  bluish-green  color  of  the  ray 
vibrating  in  the  direction  of  elongation  is  characteristic  for  it. 
It  passes  over  into  glaucophane  or  gastaldite,  which  is  a  very 
delicate  blue  in  thin  section.  The  latter  minerals  are  character- 
istic constituents  of  eclogites  and  rocks  embedded  in  them.  The 
columnar  individuals  with  a  dark  violet  blue  color  are  often 
macroscopically  visible.  They  are  best  recognized  in  thin  section 
by  the  blue  color  and  the  splendid  pleochroism.  Zonal  structure 
is  found  in  them  and  some  of  the  zones  may  have  a  very  low 
double  refraction.  It  is  not  uncommon  to  see  the  center  entirely 
colorless  with  a  low  double  refraction  and  small  extinction,  and 
surrounded  by  a  brilliant  blue  envelope.  Crocidolite,  which  is 
externally  not  unlike  glaucophane,  but  is  always  fibrous  and 
richer  in  iron,  often  occurs  as  a  secondary  development  in  crevices 
in  amphibolites  and  related  rocks.  It  is  distinguished  by  a 
comparatively  high  double  refraction.  It  has  a  larger  extinction 
angle  than  the  other  soda  hornblendes  rich  in  iron,  and  the  char- 
acter of  the  principal  zone  is  different.  The  ray  vibrating  in  the 
principal  zone  is  the  most  strongly  absorbed,  as  is  the  case  in 


DESCRIPTIVE  SECTION  293 

nearly  the  whole  hornblende  group.  It  is  remarkable  that  the 
same  phenomena  are  seen  in  crossite  and  in  the  other  soda  horn- 
blendes in  which  the  optical  plane  is  transverse  to  the  plane 
of  symmetry,  and  the  optic  normal  forms  an  angle  of  25°  or  less 
with  the  vertical  axis.  Even  in  this  case  the  principal  zone 
absorbs  the  rays  most  strongly. 

Another  series  branches  off  from  the  basaltic  hornblende. 
It  consists  of  soda  amphiboles  rich  in  ferric  iron.  Barkevikite, 
which  stands  next  to  it,  is  quite  rich  in  titanium.  It  is  brown  and 
can  only  be  distinguished  by  the  larger  extinction,  c:c  =  about 
14°.  It  is  found  especially  in  nepheline  syenites.  A  character- 
istic series  of  amphibole  minerals,  cataphorite,  seems  to  be 
related  to  barkevikite.  They  are  rarer,  have  larger  extinction, 
c:c  =  30-60°,  and  a  peculiar  reddish  color.  They  are  distin- 
guished from  all  the  other  amphiboles  by  strong  absorption 
transverse  to  the  principal  zone,  B  deep  brownish-red  to  violet, 
c  yellowish-green,  a  reddish-yellow,  B>c>tt.  Cataphorite  is 
very  frequently  surrounded  by  arfvedsonite,  which  is  free  from 
titanium  and  distinguished  by  a  bluish-green  color.  Like  the 
amphiboles  free  from  sodium,  it  shows  strong  absorption  in  the 
principal  zone,  but  this  corresponds  to  the  direction  a.  Arfved- 
sonite is  likewise  a  widespread  constituent  of  soda  rocks  and  is 
often  surrounded  by  a  border  of  aegirine. 

Pure  sodium  iron  silicate,  riebeckite,  occurs  particularly  in 
soda  granites.  The  absorption  in  the  principal  zone  is  almost 
complete  even  in  very  thin  sections.  It  forms  irregular  shreds 
that  look  quite  like  tourmaline,  and  were  formerly  determined 
as  such.  Aside  from  the  cleavage,  it  is  distinguished  by  the 
fact  that  it  absorbs  the  light  in  a  different  direction,  i.e.,  rie- 
beckite has  stronger  absorption  in  the  principal  zone.  Altera- 
tion of  this  amphibole,  like  that  of  the  pyroxenes,  leads  to  the 
formation  of  blurred  specks  rich  in  opaque  ores. 

Anthophyllite,  tremolite,  and  edenite,  as  well  as  very  weakly 
double  refracting  occurrences  of  glaucophane,  are  generally  color- 
less in  thin  section.  Actinolite,  pargasite,  and  common  green 
hornblende  are  green;  carinthine  and  arfvedsonite  are  bluish- 
green;  glaucophane,  crocidolite,  crossite,  and  riebeckite  are 
blue;  griinerite  and  cummingtonite  are  yellowish;  common 
brown  hornblende,  basaltic  hornblende,  and  barkevikite  are 
brown,  while  cataphorite  is  reddish  to  violet. 

The  extinction  angle  is  as  follows,  Fig  308,  page  288;  c:c  for 


294  PETROGRAPHIC  METHODS 

anthophyllite  and  gedrite  0°;  basaltic  hornblende  0-10°;  glau- 
cophane  4-6°;  barkevikite  and  brown  hornblende  12-14°; 
actinolite,  pargasite  15-18°;  green  hornblende  20 — 25°;  catapho- 
rite  30-60°;  crocidolite  65-70°;  arfvedsonite  and  crossite  76°; 
riebeckite  85-86°.  Unlike  the  pyroxenes  the  double  refraction 
and  the  strong  dispersion  of  the  optic  axes  decrease  in  those 
members  rich  in  ferric  iron  and  sodium. 

The  extraordinarily  manifold  properties  of  the  minerals  of  the 
amphibole  group  give  rise  to  numerous  confusions  especially  with 
the  pyroxenes.  Aside  from  the  difference  in  the  cleavage  angle, 
by  which  they  can  be  distinguished  in  basal  sections,  they  vary 
also  in  other  properties.  The  pyroxenes  have  larger  extinction, 
and  when  they  are  deeply  colored,  show  less  absorption  than  the 
amphiboles.  Tremolite  is  similar  to  sillimanite,  but  the  latter  has 
parallel  extinction  in  all  sections,  a  very  small  optic  angle,  and 
simple  cleavage  in  one  direction  transverse  to  the  principal  zone. 
It  is  also  similar  to  wollastonite  and  epidote  in  which  the  optical 
plane  is  perpendicular  to  the  principal  zone,  to  diaspore  which 
has  a  negative  principal  zone,  to  cyanite  which  is  distinguished 
by  the  acute  bisectrix  being  perpendicular  to  the  cleavage,  and 
to  lawsonite  which  always  shows  parallel  extinction  and  becomes 
cloudy  upon  heating  and  fuses  quite  easily.  Cleavage  plates  of 
the  latter  are  parallel  to  the  optical  plane.  Green  hornblende  is 
quite  like  tourmaline,  but  the  latter  has  a  stronger  absorption 
perpendicular  to  the  principal  zone.  It  also  resembles  chlorite, 
which  has  much  lower  indices  and  double  refraction,  and 
chloritoid  in  which  the  optical  plane  lies  transverse  to  the 
principal  zone.  Brown  hornblende  frequently  resembles  tour- 
maline; also  orthite,  staurolite,  and  acmite,  but  the  last  three 
are  characterized  by  higher  indices  of  refraction  and  weaker 
absorption.  It  is  also  like  biotite  which  has  lower  indices  of 
refraction,  and  also  has  the  optical  plane  transverse  to  the 
principal  zone  and  the  interference  figure  is  nearly  uniaxial  and 
appears  symmetrical  in  cleavage  plates.  Blue  hornblende  is 
also  often  confused  with  tourmaline,  and  in  rare  instances  it  is 
similar  to  chloritoid. 

Triclinic  aenigmatite,  or  cossyrite,  is  included  in  the  amphibole  group  but 
does  not  really  belong  to  it.  It  forms  prismatic  crystals  and  grains  showing 
twinning  laminations  parallel  to  the  brachypinacoid.  It  has  a  less  perfect 
cleavage  parallel  to  the  prism  making  an  angle  of  114°.  It  occurs  in  erup- 
tive rocks  rich  in  sodium.  Minute  individuals  are  found  in  the  ground 


DESCRIPTIVE  SECTION  295 

mass  of  basalts  and  are  formed  here  by  refusion  of  basaltic  hornblende. 
Hardness  is  5 . 5  and  sp.  gr.  3 . 8.  The  axial  plane  is  nearly  parallel  to  the 
brachypinacoid,  and  on  it  c:c  is  45°  f*.  On  the  macropinacoid  it  is  4°, 
2E  =  60°;  positive;  t  blackish-brown  or  almost  opaque,  B  deep  brown,  a 
lighter  reddish-brown;  very  high  indices  of  refraction.  Aenigmatite  is  at- 
tacked by  hot  hydrochloric  acid  and  fuses  easily,  forming  a  black  glass. 
It  is  similar  to  lievrite  in  both  these  properties,  but  is  more  transparent. 

Dumortierite  (11) 

Dumortierite  is  found  in  pegmatites  and  occurs  also  as  minute  needles 
or  tufts  in  certain  contact  rocks.  The  needles  are  recognized  by  their 
intense  color  and  strong  pleochroism.  It  forms  penetration  twins  according 
to  the  aragonite  law.  It  is  surrounded  by  a  pleochroic  halo  when  it  is 
included  in  cordierite.  It  gives  an  opalescent  bead  with  microcosmic 
salt.  Stronger  absorption  in  the  principal  zone  distinguishes  it  from 
piemontite  and  tourmaline,  and  the  direction  of  the  cleavage  in  basal 
sections  differentiates  it  from  hornblende.  It  is  decolorized  by  heating, 
whereas  hornblende  becomes  brown  and  fuses.  Its  distribution  has  as 
yet  not  been  investigated  to  any  great  extent. 

Axinite  (12) 

Axinite  is  a  rare  product  of  contact  metamorphism  or  of  pneumatolytic 
activity,  especially  in  coarse-grained  dikes  occurring  in  granite — Hmurite. 
If  it  has  crystal  form  the  cross  sections  are  wedge-shaped.  If  crystal  form 
is  lacking,  it  is  very  easily  overlooked  because  it  possesses  few  characteristic 
properties.  Color  and  pleochroism  are  generally  very  delicate.  It  can  be 
easily  isolated  and  then  it  gives  the  greenish  boron  flame  upon  fusing. 

Rinkite  (12) 

Rinkite  is  found  as  an  accessory  mineral  in  various  soda  rocks,  especially 
in  nepheline  syenite,  and  is  associated  with  other  rare  minerals  such  as 
mosandrite,  lavenite,  etc.,  and  is  generally  penetrated  by  fluorite.  It  is 
rapidly  decomposed  by  hydrochloric  acid  even  in  a  slide,  and  silica  con- 
taining titanium  separates  out.  It  fuses  easily  with  intumescence  and  gives 
off  fluorine.  The  positive  bisectrix  is  nearly  perpendicular  to  the  cleavage 
plates,  this  being  the  best  property  to  use  for  its  determination.  It  is 
often  transformed  into  yellowish,  clouded  aggregates. 

Sillimanite  (12) 

Sillimanite  forms  colorless  needles  without  terminations,  and 
these  are  often  finely  shredded  and  have  a  silky  luster  macro- 
scopically.  Radial  aggregates,  that  are  only  transparent  in  thin 
section,  are  called  fibrolite.  It  is  found  in  contact  rocks  in  heli- 
coidal  bands  wound  in  various  ways  and  penetrating  through 

*f.-  front. 


296 


PETROGRAPHIC  METHODS 


the  other  constituents  of  the  rocks.  Elongated  sections  are 
narrow  lath-shaped  and  often  bent,  or  fractured  transversely. 
^  Basal  sections  only  show  good  forms 
where  the  individuals  are  isolated. 
They  are  then  extremely  characteristic, 
Fig.  311.  It  is  disseminated  principally 
in  contact  rocks  rich  in  aluminium,  in 
certain  gneisses,  and  as  isolated  needles 
in  granite.  Alteration  is  not  known. 


\ 

I 

\ 

/' 

\ 

/ 

\ 

/ 

\ 

/ 

\ 

f 

1 

I 

\ 

1 

\ 

1 

\ 

I 

\ 

I 

\ 

(010? 


s~  (      VH* 

(}Jv        v^> -X 


(110) 


FIG.  310. 


(100)^ 
FIG.  311. 


Sillimanite,  Section 
Parallel  (100).  Parallel  (001). 


C  M  (001) 


(100) 


Concerning  the  microscopic  distinction  from  apatite  or  trem- 
olite,  see  those  minerals.  The  positive  character  of  the  principal 
zone  distinguishes  it  from  andalusite 
and  prismatine,  parallel  extinction  from 
chloritoid,  as  also  the  position  of  the 
optical  plane  parallel  to  the  cleavage, 
and  the  absence  of  twinning  laminations. 

Datolite  (12) 

Datolite  is  only  known  as  a  secondary 
formation,  chiefly  in  cavities  in  basic  eruptive 
rocks.  Its  distribution  as  a  rock  constituent 
has  not  been  investigated,  but  it  may  occur  in 
certain  contact  rocks.  It  forms  granular  or 
fibrous  aggregates  parallel  to  the  6  axis  — 
botryolite.  In  the  latter  the  axial  plane  lies 
transverse  to  the  fibers.  It  fuses  before  the 
blowpipe  to  a  clear  glass  and  colors  the  flame 
green.  The  powder  has  a  strong  alkaline 

reaction  even  before  fusing.     Datolite  is  best  determined  by  separating  it 
from  the  rock  powder  with  heavy  liquids,  its  specific  gravity  being  2.9, 


FIG.  312. — Datolite, 

Section  Parallel  Plane  of 

Symmetry. 


DESCRIPTIVE  SECTION  297 

and  then  treating  it  with  hydrochloric  acid.     Alcohol  is  then  poured  upon 
the  residue  and  if  datolite  was  present  it  will  burn  with  a  greenish  flame. 

Mosandrite  (12) 

Mosandrite  is  found  as  an  accessory  constituent  in  nepheline  syenite 
in  small  tabular  crystals  associated  with  rinkite,  etc.,  and  penetrated 
by  fluorite.  It  is  brittle  and,  therefore,  generally  checked.  Lath-shaped 
cross  sections  show  twinning  laminations.  The  mineral  is  often  changed 
into  earthy  brown  to  opaque  aggregates.  Johnstrupite  is  very  similar  to 
it  but  has  lower  indices  of  refraction.  Mosandrite  is  easily  fusible  and 
forms  a  light  colored  bead.  It  gives  a  dark  red  solution  with  hydrochloric 
acid  and  evolves  chlorine.  Strong  inclined  dispersion  of  the  axes  in  cleavage 
plates  distinguishes  it  from  rinkite. 

Barite  (Heavy  Spar)   (12) 

Barite  forms  granular  or  platy,  rarely  fibrous,  aggregates  without  dis- 
tinct borders.  They  occur  now  and  then  filling  crevices  and  cavities  in 
rocks,  as  cement  in  sandstone,  and  particularly  as  concretions  in  marl  and 
sand.  These  concretions  are  frequently  filled  with  inclusions  of  sand,  etc., 
and  consist  of  imperfect,  tabular  crystals  arranged  in  rosettes.  If  it  forms 
the  predominant  cement  of  a  rock,  it  is  recognized  by  the  great  weight  of 
the  same.  Cleavage  plates  of  the  mineral  are  perpendicular  to  the  negative 
obtuse  bisectrix.  If  it  occurs  in  subordinate  amounts,  it  is  easily  over- 
looked. Its  distribution  as  a  rock  constituent  is,  therefore,  not  accurately 
known.  Aside  from  a  different  flame  reaction,  it  is  distinguished  from 
celestite  by  a  comparatively  small  optic  angle  and  from  quartz,  feldspar, 
and  similar  colorless  rock  constituents  by  higher  indices  of  refraction  and 
insolubility  in  hydrofluoric  acid. 

Andalusite  (12) 

Andalusite  is  generally  found  in  short,  prismatic,  rounded 
crystals  in  which  very  frequently  the  cleavage  cannot  be  recog- 
nized. The  individuals  tend  to  arrange  themselves  in  divergent, 
radial  groups  or  fingered  aggregates.  It  is  colorless  in  thin 
section  or  speckled  with  a  pale  reddish  color,  and  is  then  pleo- 
chroic.  It  is  found  now  and  then  in  parallel  intergrowths  with 
sillimanite.  The  arrangement  of  graphite  inclusions  is  especially 
characteristic — chiastolite,  Figs.  313  and  314,  also  Fig.  211,  page 
196. 

Andalusite  is  very  frequently  altered  to  a  compact  aggregate 
of  mica,  but  these  pseudomorphs  can  be  recognized  by  the 
arrangement  of  the  graphite  inclusions.  If  this  arrangement 
becomes  more  and  more  irregular  and  the  outer  form  quite 
imperfect,  these  elongated  mica  aggregates  filled  with  graphite 


298 


PETROGRAPHIC  METHODS 


dust  are  extremely  difficult  to  determine.  They  are  widely 
distributed  in  contact  rocks  and  form  a  large  part  of  the  knots 
that  appear  macroscopically  in  such  rocks.  In  most  favorable 
cases  a  few  unaltered,  but  corroded  residues  point  to  the  original 
andalusite.  Its  chief  distribution  is  in  contact  rocks  rich  in 
aluminium.  Andalusite  is  the  lightest  of  the  three  modifications 


(100) 


Andalusite,  Section  Parallel  to 
(001)  (010) 


of  aluminium  silicate.  It  is,  therefore,  absent  in  rocks  which 
have  been  formed  under  high  pressure,  especially  in  the  contact 
rocks  of  the  Central  Alps.  * 

The  negative  character  of  the  principal  zone  affords  a  dis- 
tinction of  colorless  andalusite  from  sillimanite,  and  the  lower 
double  refraction  together  with  the  orthorhombic  properties  of 
all  sections  distinguishes  it  from  diopside.  Red  andalusite  may 
appear  similar  to  hypersthene  but  they  have  opposite  characters 
of  the  principal  zone.  It  may  also  be  confused  with  piemontite 
in  which  the  optical  plane  lies  transverse  to  the  cleavage. 


Lazulite  (12) 

The  petrographical  importance  of  lazulite  has  been  very  little  in- 
vestigated. Good  crystals  of  it  have  been  found  in  quartz  veins  in  phyllites 
and  in  impregnations  in  granular  limestone — bluespar.  High  double  re- 
fraction, absence  of  cleavage,  and  its  pyramidal  form  distinguish  it  from 
other  blue  minerals. 


DESCRIPTIVE  SECTION  299 

Carpholite  (12) 

Carpholite  is  known  as  a  fibrous,  secondary  formation  in  crevices  in  altered 
rocks.  Weaker  absorption  in  the  principal  zone  distinguishes  it  from 
most  other  yellowish  minerals  except  tourmaline,  which  is  uniaxial,  and 
epidote  in  which  the  optical  plane  lies  transverse  to  the  cleavage. 

Prehnite  (12) 

Prehnite  belongs  to  those  minerals  whose  petrographical  im- 
portance is  but  little  known.  It  is  found  in  radial,  flaky  aggre- 
gates in  cavities  in  eruptive  rocks,  or  in  compact  aggregates 
as  a  constituent  of  saussurite, — lotrite.  It  is  also  quite  wide- 
spread as  an  alteration  product  of  feldspar.  No  other  mineral 
shows  such  variable  properties  as  prehnite,  and  it  can  only  be 
identified  by  the  rosette  structure  in  thin  sections,  which  is  ex- 
tremely characteristic.  Double  refraction,  normal  and  anom- 
alous interference  colors,  position  of  the  optical  plane,  size 
of  the  optic  angle,  and  the  lattice  structure  vary  in  the  dif- 
ferent occurrences,  so  that  even  though  the  mineral  can  be  de- 
termined macroscopically  it  presents  many  difficulties  under  the 
microscope. 

Celestite  (12) 

Celestite  is  analogous  to  barite  as  a  rock-forming  mineral,  except  that  it 
rarely  forms  individuals  so  rich  in  inclusions.  See  under  barite  for  its 
distinction  from  that  and  other  similar  minerals. 

Aragonite  (13) 

Aragonite  is  a  rare  constituent  of  rocks,  but  it  occurs  in  columnar  aggre- 
gates which  are  very  difficult  to  distinguish  from  fibrous  calcite.  The 
principal  characteristics  by  which  it  can  be  determined  when  accurate 
observation  of  the  interference  figure  is  not  possible,  are  the  lack  of  cleavage 
and  the  special  reaction  with  cobalt  solution,  page  175.  It  is  probably  quite 
widespread  as  an  original  constituent  of  rocks,  but  it  has  been  very  ex- 
tensively transformed  into  calcite.  Aragonite  itself  is  found  only  in 
secondary  deposits  from  hot  springs,  particularly  as  onyx. 

Wollastonite  Group  (13) 

Wollastonite  forms  columnar  to  flaky  individuals  in  which  the  axis  of 
symmetry  is  the  axis  of  elongation.  Cross  sections  parallel  to  it  are  lath- 
shaped,  and  those  perpendicular  to  it  are  somewhat  elongated  along  the 
orthopinacoid  and  are  bounded  by  from  six  to  eight  faces.  The  cleavage 
cracks  are  all  parallel  to  the  axis  of  symmetry,  and  in  transverse  sections 
two  systems  of  very  perfect  cleavage  cracks  nearly  at  right  angles  are  seen 


300 


PETROGRAPHIC  METHODS 


(101) 


c  (ooi) 


(100) 


along  with  another  set  that  is  less  distinct,  Fig.  315.  Twinning  parallel 
to  the  orthopinacoid  is  very  common.  It  is  recognized  in  transverse  sec- 
tions by  the  extinction.  Wollastonite  is  chiefly  characteristic  for  granular 
limestone.  In  feldspar-bearing  rocks  it  is  only  found  in  the  neighborhood 
of  such  formations.  When  it  occurs  in  eruptive  rocks,  it  is  probably  the 
result  of  resorbed  lime  inclusions  or  it  may  form  pseudomorphs  after  an- 
orthite.  The  moistened  powder  gives  an  alkaline  reaction.  It  fuses  be- 
fore the  blowpipe  to  a  crystalline  slag.  The  ease  with  which  it  is  attacked 
by  hydrochloric  acid  distinguishes  it  from  numerous  other  minerals  with 

which  it  might  be  confused  if  the  cross  sec- 
tions are  poorly  developed.  The  mineral  is 
always  colorless  but  is  well  characterized  by 
the  position  of  the  optical  plane  transverse 
to  the  cleavage  in  sections  showing  parallel 
extinction,  the  comparatively  small  optic 
angle  with  distinct  inclined  dispersion,  and 
other  optical  and  chemical  properties.  The 
lower  indices  of  refraction  and  the  solubility 
distinguish  it  from  epidote  minerals  which 
are  similarly  orientated. 

Pectolite   is   related   to   wollastonite.     It 
occurs   in   fibrous,    radial  aggregates   as   a 
Section  Parallel  Plane  of  Symmetry,  secondary  formation  in  crevices  of  eruptive 

rocks    rich    in    lime    and    in   amphibolites 

derived  from  them,  and  now  and  then  as  an  alteration  product  of  feldspar 
in  soda  rocks.  It  is  distinguished  from  wollastonite  by  the  orientation 
of  the  optical  plane  parallel  to  the  positive  principal  zone  and  the  higher 
double  refraction.  Cleavage  plates  are  almost  perpendicular  to  the  obtuse 
bisectrix. 

Rosenbuschite  is  still  rarer.  It  forms  colorless  to  yellowish,  radial,  fibrous 
aggregates  in  nepheline  syenites.  In  it  the  optical  plane  is  parallel  to  the 
principal  zone  which  is  negative.  It  is  characteristically  accompanied  by 
fluorite.  It  is  distinguished  from  similar  minerals  by  the  negative  character 
of  the  principal  zone  and  by  its  solubility. 


FIG.  315.— Wollastonite, 


Humite  Group  (13) 

The  minerals  of  the  humite  group  cannot  be  distinguished  from  one  an- 
other macroscopically  and  frequently  also  microscopically.  They  are  found 
mostly  in  contact  limestones  or  dolomites  in  rounded  grains  in  which  the 
cleavage  is  very  poorly  developed.  They  often  show  irregular  cracks. 
Twinning  lamination  parallel  to  the  basal  pinacoid  is  common  in  chondro- 
dite  and  clinohumite.  If  the  humites  are  colorless,  they  are  very  similar 
to  olivine  from  which  they  can  only  be  distinguished  by  the  extremely 
low  dispersion  and  by  the  position  of  the  optical  plane,  which  is  parallel  to 
the  cleavage  in  humite.  The  direction  of  extinction  in  the  monoclinic 
members,  showing  cleavage  cracks  or  twinning  lamination  also  serves  to 
distinguish  them  from  olivine.  See  under  olivine  for  the  distinction  between 
colored  varieties  and  titanium  olivine.  The  humites  often  pass  over  into 


DESCRIPTIVE  SECTION 


301 


serpentine  with  the  formation  of  the  mesh  structure.  Brucite  also  results 
from  such  alterations.  They  become  white,  but  do  not  fuse  before  the  blow- 
pipe. 


(001)      C  =/ 


(001) 


FIG.  316.— Humite, 


FIG.  317. — Clinohumite, 


FIG.  318.— Chondrodite, 


Section  Parallel  (010). 

Topaz  (13) 

Topaz  forms  short,  prismatic  crystals  and  grains  which  show 
perfect  cleavage  in  the  elongated  sections  by  the  presence  of  a 
few  very  sharp  cracks.  Cleavage  plates  are  perpendicular  to  the 
acute,  positive  bisectrix.  The  optic  angle  is  generally  quite  large, 
but  is  variable.  The  mineral  is  also  found  in  columnar  or  radial 
aggregates — pycnite — where  it  has  been  formed  from  other 
minerals  by  the  activity  of  fumaroles,  as  in  the  topaz  quartz 
porphyries,  occurring  in  connection  with  tin  ore  veins.  In  these 
the  character  of  the  principal  zone  is  positive.  The  mineral  is 
known  in  the  associated  granites  as  a  primary  constituent,  but 
otherwise  it  is  always  secondary  and  points  to  very  intensive 
post-volcanic  processes. 

Topaz  is  always  colorless  in  thin  section  and  is  distinguished 
from  quartz  by  higher  relief.  Distinction  from  sillimanite  may 
be  very  difficult,  but  the  latter  is  characterized  by  higher  indices 
and  double  refraction,  and  by  the  cleavage  parallel  to  the  prin- 
cipal zone.  Topaz  can  be  easily  isolated  from  a  rock  on  account 
of  its  high  specific  gravity  and  resistance  to  acids.  It  is  infusible 
before  the  blowpipe,  but  turns  white  and  at  the  same  time 
flakes  up. 

Anhydrite  (14) 

Anhydrite  itself  forms  a  rock  consisting  of  granular,  or  less 
often  columnar  aggregates,  that  change  easily  into  gypsum.  It 
is  often  colored  blue  or  red,  macroscopically,  but  in  thin  section 


302  PETROGRAPHIC  METHODS 

it  is  always  colorless  and  has  three  cleavages  differently  devel- 
oped. Its  solubility  in  water  is  characteristic,  and  gypsum 
crystallizes  out  upon  evaporation.  It  is  very  similar  to  musco- 
vite  in  thin  section  but  the  latter  has  but  one  distinct  cleavage. 
The  mineral  is  confined  to  sedimentary  rocks  and  is  often  found 
in  very  coarse-grained  aggregates  in  the  crevices  of  such  rocks. 

Mica  Group  (14) 

A  very  large  series  of  minerals  is  characterized  by  an  excep- 
tionally perfect  cleavage  in  one  direction,  so  that  they  have  a 
flaky  or  scaly  appearance.  The  outer  form  of  the  small  plates  is 
almost  without  exception  hexagonal.  A  table  of  the  members 
of  this  group,  that  are  important  in  rocks,  can  be  found  on  page 
270.  The  mica  group  is  the  most  important  of  these. 

The  members  of  the  mica  group,  which  are  important  in  rocks, 
are  potassium  mica— muscovite,  magnesium  micas — phlogopite 
and  biotite,  and  the  lithium  micas,  "which  are  classed 
together  under  the  name  lithionite.  The  determination  of  the 
crystal  system  of  mica  is  generally  not  possible  by  optical 
methods.  Most  of  them  show  parallel  extinction  although  they 
are  monoclinic,  and  apparently  uniaxial  varieties  are  very 
common.  If  an  optic  angle  can  be  determined  distinctly,  it 
may  be  observed  that  the  negative,  acute  bisectrix  coincides 
almost  exactly  with  the  vertical  axis,  while  in  the  uniaxial 
varieties  this  is  the  direction  of  the  optic  axis. 

Cleavage  plates  of  mica  are  perpendicular  or  almost  perpen- 
dicular to  the  acute,  negative  bisectrix.  The  true  optic  angle  is 
small  to  medium  large,  from  approximately  0°  in  most  magnesium 
micas  to  30-40°  in  normal  muscovite,  but  rarely  larger.  In  con- 
vergent light,  therefore,  the  optic  axes  appear  symmetrically 
within  the  field  of  vision.  Transverse  sections  give  brilliant 
interference  colors  in  consequence  of  the  high  double  refraction, 
and  they  extinguish  almost  exactly  parallel  to  the  numerous, 
sharp  cleavage  cracks.  When  such  sections  are  colored,  they 
are  distinctly  pleochroic.  Basal  sections  are  entirely  different, 
making  it  difficult  for  the  beginner  to  determine  it.  This  varia- 
tion aids  the  experienced  observer,  however,  to'  recognize  it. 
Such  sections  are  either  entirely  dark  between  crossed  nicols, 
or  they  show  low  interference  colors  corresponding  to  the 
small  difference  ;--/?,  which  never  exceeds  0.008.  In  addition 
to  this,  nothing  is  seen  of  the  cleavage,  and  the  colored  members 


DESCRIPTIVE  SECTION 


303 


of  the  group  are  rarely  pleochroic.  However,  in  convergent 
polarized  light  a  remarkably  distinct  interference  figure  is  always 
obtained. 

The  optical  plane  sometimes  lies  perpendicular  to  the  plane  of 
symmetry — mica  of  the  first  order,  Fig.  319,  and  sometimes  in 
the  plane  of  symmetry — mica  of  the  second  order,  Fig.  320.  Musco- 
vite belongs  to  the  first  type  and  most  of  the  biotites  to  the  second. 
The  orientation  can  be  recognized  by  means  of  the  percussion 
figure  which  is  shown  in  the  figures  for  all  the  minerals  belonging 
in  this  group.  Twinning  is  very  widespread  and  generally 
parallel  to  the  basal  pinacoid.  It  is  not  often  observed  in 


Mica  Orientated  by  Percussion  Figure. 
First  Order.  Second  Order. 


transverse  sections  because  of  the  parallel  extinction,  but  it  can 
be  recognized  in  thick  cleavage  plates  by  means  of  the  distorted 
interference  figure.  Micas  are  elastic  and  are  perfectly  flexible 
under  the  influence  of  orogenic  stresses.  They,  therefore, 
occur  in  variously  bent  forms,  Fig.  204,  page  192.  Now  and  then 
they  are  torn  apart  parallel  to  the  gliding  plane,  which  is  trans- 
verse to  the  cleavage,  and  the  alteration  to  chlorite  proceeds 
outward  preferably  from  such  parting  structures. 

Colored  micas  always  show  great  differences  in  absorption,  and 
the  browns  possess  this  to  a  higher  degree  than  the  rarer  greens. 
The  rays  vibrating  in  the  cleavage  plane  are  always  the  most 
strongly  absorbed,  and  generally  show  no  difference  among 
themselves.  For  this  reason  cleavage  plates  are  generally  not 
pleochroic.  Biotites  with  entirely  anomalous  properties  occur  in 
soda  rocks,  but  they  are  very  rare.  Here,  they  have  quite  a  large 
optic  angle  and  quite  an  appreciable  angle  of  extinction  and 
also  show  a  distinct  difference  in  absorption  between  ft-  and  c. 


304  PETROGRAPHIC  METHODS 

In  pleochroic  halos,  which  are  found  in  the  micas  in  all  rocks, 
the  direction  of  the  stronger,  often  complete  absorption,  is  like- 
wise parallel  to  the  cleavage  cracks.  Indices  of  refraction  and 
double  refraction  are  higher  in  them. 

The  micas  always  contain  the  hydroxyl  group.  They  are  more 
widespread  among  the  intrusive  rocks  than  among  the  extrusive. 
In  the  latter  they  are  often  magmatically  resorbed  in  a  manner 
analogous  to  that  described  for  hornblende.  The  specific  gravity 
is  high  but  they  cannot  be  isolated  by  heavy  liquids  on  account 
of  their  flaky  development.  See  page  155  for  a  method  of  sepa- 
rating mica  from  other  minerals.  Members  rich  in  iron  are 
easily  attracted  by  an  electro-magnet.  Light  colored  micas  are 
only  slowly  attacked  by  hydrofluoric  acid,  but  the  darker  ones 
are  decolorized  by  hydrochloric  acid  and  gradually  lose  their 
double  refraction.  The  fusibility  is  quite  variable  from  the 
difficultly  fusible  muscovite  to  the  easily  fusible  lithia  micas.  It 
may  also  be  noted  that  the  micas  like  all  aluminium  silicates  be- 
come blue  when  roasted  with  cobalt  nitrate,  and  this  is  the 
only  real  positive  reaction,  without  making  thorough  chemical 
tests,  for  distinguishing  between  mica  and  talc. 

Muscovite,  Fig.  321,  is  rare  as  a  primary  constituent  of  eruptive 
rocks.  It  occurs  in-very  large  crystals  in  granitic  .pegmatites 

and    forms    irregular    flakes    in 
\$Pg  '1  binary  granites,  which  are  recog- 

nized  macroscopically  by  the 
brilliant  luster.  It  is  also  found 
abundantly  in  fresh  plagioclase 
in  the  central  granite.  Here  it 
forms  minute,  well-defined,  and 
;  FIG.  32i.— Muscovite,  Orthopinacoid.  regularly  arranged  scales,  which 

can  only  be  considered  as  primary 

inclusions.  The  appearance ,.  of  secondary  mica,  which  may 
develop  from  any  of  the  feldspars,  is  entirely  different.  It  is 
sometimes  in  larger  scales,  which  follow  the  cleavage  cracks, 
etc.,  and  form  veins  or  bands  with  irregular  properties.  More 
often  it  is  a  dense,  confused,  scaly  formation  imbedded  in  the 
clouded  substance  of  the  feldspar  or  entirely  replacing  it. 
Similar  aggregates,  that  are  macroscopically  always  dense, 
occur  as  pseudomorphs  after  nepheline,  scapolite,  andalusite, 
etc.  Their  properties  indicate  that  they  belong  to  the  sericites 
described  below.  Small  flakes  of  muscovite  are  found  also  in 


DESCRIPTIVE  SECTION 


305 


the  ground  mass  of  quartz  porphyries  that  have  been  somewhat 
altered  and  they  can  only  be  secondary. 

Muscovite  is  far  more  widespread  in  contact  rocks,  especially 
those  formed  by  piezocontact  metamorphism.  It  is  shown  in 
the  Allgemeine  Gesteinskunde,  page  138,  how  mica  schist  occurs 
under  these  conditions  in  place  of  the  normal  hornfels.  Mus- 
covite is  the  most  frequent  cause  of  the  schistose  structure  of  the 
crystalline  schists  formed  by  contact  metamorphism.  When 
light  mica  occurs  in  isolated  individuals  that  are  poorly  developed 
but  are  macroscopically  distinct,  the  schistosity  is  generally 
very  imperfect.  This  phenomenon  is  seen  in  many  injected 
schists,  especially  in  amphibolite  and  eclogite,  but  in  the  latter 
rock  the  mica  does  not  appear  to  belong  to  muscovite,  but  is  a 
mica  whose  properties  have  not  been  accurately  determined. 

The  mica  in  these  rocks  tends  to  bind  itself  into  membranes 
with  a  silvery  luster,  which  wrap  around  all  the  other  rock  con- 
stituents as  in  a  mica  schist. 
In  the  phyllites  these  mem- 
branes become  finer  and  finer 
and  are  folded  in  various 
ways,  Fig.  322.  Then  the 
single  flakes  cannot  be  seen 
with  the  naked  eye  and  the 
mica  can  only  be  recognized 
by  the  silky  luster  of  the 
rock.  Such  fine,  scaly  aggre- 
gates of  mica,  related  by  a 
series  of  transition  members 
to  true  muscovite,  are  called 
sericite.  They  have  properties 
different  from  muscovite,  chief 
among  which  are  the  smaller 
optic  angle  and  a  greater  susceptibility  to  the  action  of  acids. 

This  mineral  forms  the  principal  constituent  of  a  widespread 
series  of  sericite  schists,  that  have  extremely  thin  layers,  and 
are  mostly  white  with  a  silky  luster.  They  are  rocks  in  which  a 
few  phenocrysts  of  corroded  quartz  can  be  distinctly  seen,  Fig. 
199,  page  189.  These  can  only  be  looked  upon  as  altered  quartz 
porphyry.  Granite,  gneiss,  etc.,  suffer  similar  sericitization  now 
and  then,  especially  in  the  neighborbood  of  faults  and  when 
certain  ore  veins  are  formed  in  the  crevices  thus  produced. 
20 


FIG.  322. — Folded  Sericite  Membrane. 
Quartz  Phyllite,  Sunk,  Steiermark. 


306  PETROGRAPHIC  METHODS 

Rocks  rich  in  sericite  are  apparently  quite  similar  to  fine, 
scaly  talc,  and  this  is  increased  by  the  greasy  feeling  of  them. 
It  is  interesting  to  note  that  nearly  all  such  occurrences  were 
formerly  designated  as  talc  schists,  and  even  to-day,  sericite 
schists  are  generally  included  with  talc  schists.  Aggregates  of 
soda  mica — paragonite — are  outwardly  very  similar  to  sericite. 
They  are  well-known  because  of  the  content  of  staurolite  and 
cyanite.  They  replace  the  granite  pegmatites  in  the  Central 
Alps.  Nothing  is  known  of  a  further  distribution  of  this  mineral. 
Dense  light  green  or  yellowish  aggregates  of  mica  with  properties 
similar  to  soapstone  are  known  in  small  masses  in  the  schists  of 
the  Central  Alps  and  in  other  places  under  analogous  conditions. 
They  are  called  onkosine,  pragratite,  cossaite,  damourite,  margar- 
odite,  etc.,  and  consist  of  aggregates  that  are  composed  of  various 
sorts  of  minerals  which  cannot  be  distinguished  from  sericite. 

Muscovite  is  always  a  mica  of  the  first  order  but  is  often  in 
parallel   intergrowth   with   biotite.     Characteristic   intergrowth 
with  quartz  resembling  palm  leaves  occurs  in  pegmatitic  rocks 
and  called  mica  palme.     Alteration  of  muscovite  is  not  known. 
It  is   therefore   often  found   unaltered  in 
clastic  rocks,  but  without  accurate  investi- 
gation it  can  be  confused  with  bleached 
biotite. 

Phlogopite,  Fig.  323,  is  a  typical  mineral 
of  contact-metamorphosed  limestones  and 
dolomites,  and  occurs  principally  in  rounded 
crystals  or  those  elongated  along  the  verti- 
FIG.  323.  cal   axis.      Macroscopically,   it   is   usually 

Biotite  (Phlogopite),  T    i  ,    i  -r  .    i      * 

Cleavage  Plate.  Ver7  ljgnt  brown.     Large  tabular  crystals 

of  commercial  importance  are  found  in  the 

granite  pegmatites,  which  cut  through  carbonate  rocks.  It  is 
always  colorless  in  thin  section  and  distinguished  from  muscovite 
by  the  fact  that  it  is  usually  nearly  uniaxial,  and  that  the  cleavage 
cracks  do  not  have  a  tendency  to  be  so  sharp.  Oblique  extinc- 
tion is  observed.  Needle-like  inclusions — sillimanite  and  rutile 
— tend  to  arrange  themselves  diagonal  to  the  rays  of  the  per- 
cussion figure  and  upon  alteration,  which  occurs  now  and  then, 
sagenitic  intergrowths  of  rutile  are  seen  along  with  scaly  aggre- 
gates. The  distribution  of  phlogopite  in  rocks  is  but  little 
known. 

Biotite   is   much   more   widespread.     It   is   macroscopically 


DESCRIPTIVE  SECTION  307 

brown  to  black  and  often  has  a  bronzy  luster.  Dark  green 
varieties  are  rarer.  Deep  reddish-brown  rubellane  of  the  basic, 
extrusive  rocks  is  generally  burned  and  strongly  resorbed  by  the 
magma.  A  few  rarer  varieties  are  the  micas  of  the  first  order — 
anomite — and  these  frequently  have  an  extinction  of  4-5°,  and 
a  somewhat  larger  optic  angle.  Most  of  the  biotites  are  micas 
of  the  second  order — meroxene  and  lepidomelane.  The  latter  is 
particularly  rich  in  iron  and  titanium,  and  the  absorption  of  the 
rays  vibrating  parallel  to  the  cleavage  is  almost  complete  in 
thin  sections.  In  rarer  varieties,  which  occur  especially  in  the 
group  of  soda  rocks,  the  optic  angle  is  quite  large,  up  to  35°, 
and  at  the  same  time  an  important  extinction,  up  to  10°,  makes 
its  appearance.  Transverse  sections  show  distinct  twinning 
laminations  and  a  difference  between  the  two  rays  vibrating 
nearly  in  the  cleavage  plane,  B  golden  yellow,  c  reddish-brown. 

Most  of  the  biotites  are  brown  in  thin  section  with  different 
degrees  of  intensity.  Less  often  they  are  colored  yellowish- 
brown,  red  brown  or  finally  green,  a  yellowish,  &  =  c  green. 
Biotite  is  widely  disseminated  in  eruptive  rocks  and  contact 
rocks  of  all  sorts.  The  best  developed  crystals,  apparently 
hexagonal,  are  found  in  glassy  extrusive  rocks  while  in  other 
extrusive  rocks  they  are  magmatically  resorbed.  They  are  the 
more  resorbed  the  more  crystalline  the  rock.  Inclusions  of 
apatite  and  zircon  are  frequent  and  the  former  are  without,  the 
latter  with  pleochroic  halos,  Fig.  216,  page  197.  Sagenitic  inter- 
growths  of  rutile  are  also  common.  In  granulitic  rocks  the  cross 
sections  are  often  completely  perforated  by  rounded  grains  of 
quartz.  The  flakes  themselves  have  rounded  outline  in  contact 
rocks,  and  skeletal  development  is  occasionally  observed  as  well. 
They  do  not  combine  to  form  membranes  as  frequently  as 
sericite  does.  They  never  form  such  fine  crystalline  films.  On 
the  other  hand,  biotite  flakes  pentrate  the  other  constituents  of  a 
hornfels  in  a  helicoidal  manner  and  in  schistose  rocks,  the  flakes 
are  often  placed  transverse  to  the  schistosity.  They  are  then 
often  filled  with  graphite,  etc.,  and  do  not  show  a  thin  tabular  de- 
velopment, but  often  a  distinct  prismatic  growth,  Fig.  324,  in 
contradistinction  to  the  biotite  flakes  which  lie  in  the  structure 
plane  of  the  rock. 

It  is  found  in  parallel  intergrowth  with  muscovite  in  binary 
granites,  gneisses,  etc.,  and  with  chlorite  in  the  central  granites. 
Perfect  freshness  of  the  biotite  prevents  confusion  with  the 


308 


PETROGRAPHIC  METHODS 


alteration  of  mica  to  chlorite,  which  is  so  common.  It  is  often 
intergrown  with  hornblende  and  pyroxene,  or  a  narrow  homo- 
geneous border  surrounds  an  irregular  grain  of  opaque  ore,  par- 
ticularly in  gabbro. 

Alteration  is  very  common.  The  simplest  process  is  bleaching 
in  which  the  mineral  loses  its  color  and  fresh  appearance,  and 
passes  over  into  a  colorlesp  mica  with  a  somewhat  lower  double 

refraction.  Chloritization  is 
more  frequent.  Proceeding 
inward  from  the  edges  and 
along  gliding  planes,  the  min- 
eral is  changed  to  lamellar 
chlorite.  The  separation  of  ti- 
tanium minerals,  then  quartz, 
carbonates,  iron  ores,  epidote, 
etc.,  allow  the  process  of 
alteration  to  be  followed 
closely.  Sometimes  rutile  is 
observed  in  the  form  of 
sagenite,  the  crystals  inter- 
secting each  other  at  an  angle 
of  60°.  This  rutile  is  a  by- 
product of  the  formation  of 
chlorite  or  the  bleaching  process.  In  certain  cases  almandine 
or  olivine  are  altered  into  biotite. 

The  lithionites  are  sometimes  light,  and  sometimes  dark  col- 
ored. They  are  constituents  of  lithionite  granite  and  can  only 
be  distinguished  from  muscovite  or  biotite  by  chemical  reactions, 
especially  the  flame  reaction,  and  by  the  greater  fusibility.  Dark 
lithia  micas  have  a  variable  optic  angle  up  to  about  65°,  and  are 
micas  of  the  second  order.  They  are  observed  first  of  all  in  the 
lithionite  granites,  which  occur  in  combination  with  tin  ore  veins, 
and  they  are  here  the  principal  micas.  The  colorless,  as  well  as  the 
brown,  are  often  completely  filled  with  pleochroic  halos,  which 
occur  around  inclusions  of  rutile,  zircon,  and  cassiterite.  Mica 
very  rich  in  iron,  macroscopically  black — raven  mica — is  found 
in  certain  soda  rocks. 

See  the  introduction  to  this  section  for  a  distinction  between 
the  real  micas  and  a  series  of  minerals  with  a  similar  development. 
Special  attention  is  called  to  the  use  of  the  cleavage  plates  in 
the  investigation,  for  this  is  sometimes  the  only  means  of  dis- 


FIG.  324. — Biotite,  Transverse  to  Schistocity, 

Prismatic.     Graphite  Schist,  Maurertal, 

Grosvenediger. 


DESCRIPTIVE  SECTION  309 

tinction,  e.g.,  muscovite  from  pyrophyllite,  although  the  latter 
has  a  much  larger  optic  angle.  Therefore  the  phenomena,  which 
the  cleavage  plates  of  mica  and  micaceous  minerals  show,  are 
considered  first  in  the  diagrams;  see  also  the  table,  page  270. 
This  method  on  the  other  hand  does  not  give  results  m  the  case 
of  dense  aggregates  of  talc.  Here  chemical  reaction,  page  168, 
must  find  a  place.  Diaspore  may  appear  similar  to  colorless 
micas  on  account  of  the  coincidence  of  the  interference  colors, 
but  it  is  characterized  by  higher  indices  of  refraction.  Also 
anhydrite,  which  has  more  than  one  cleavage  not  all  equal,  is 
similar  to  colorless  mica.  The  most  perfect  cleavage  planes  of 
that  mineral  are  perpendicular  to  the  obtuse  negative  bisectrix. 
Minerals  of  the  amphibole  group,  tourmaline,  and  orthite  are 
confused  with  colored  micas,  but  all  these  minerals  have  higher 
indices  of  refraction  and  poorer  cleavages.  The  hydrofluosilicic 
acid  reaction,  page  163,  is  valuable  under  all  circumstances  for 
determining  the  micas. 

Chrome  mica—fuchsite — must  also  be  mentioned.  It  occurs  in  emerald 
green  flakes  in  contact  rocks.  It  has  a  variable  optic  angle,  a  pale  sky- 
blue,  B  =  c  siskin  green. 

Seladonite  is  appended  to  the  mica  group.  It  is  an  alkali  silicate  similar 
to  mica  and  occurs  as  a  rock  constituent  in  basic  eruptive  rocks  and 
tuffs.  It  is  sometimes  in  the  veins  in  them,  sometimes  as  pseudomorphs 
after  augite,  and  sometimes  as  an  impregnation  in  the  rock.  It  is  dark  green 
in  color,  has  dense,  earthy  properties — green  earth — and  is  very  soft. 
Sp.  gr.  =  2.6-2.8.  It  is  apparently  uniaxial,  negative,  o  pale  yellowish- 
green,  c  deep  green.  Double  refraction  is  somewhat  lower  than  that  of 
the  micas.  Glauconite  with  a  similar  composition  is  distinguished  from  it 
by  its  occurrence,  which  appears  to  be  confined  to  the  sedimentary  rocks — 
green  sand.  It  is  found  in  small  dark  green  spheres,  which  appear  macro- 
scopically  like  grains  of  gun  powder  and  is  often  collected  into  masses  as  an 
agent  of  petrifaction,  especially  of  foraminifera.  Its  structure  is  generally 
confused  even  microscopically.  Distinct  flaky,  radial  aggregates,  showing 
marked  pleochroism  from  light  yellow  to  deep  green,  are  rare.  It  is  also 
negative  but  is  biaxial,  2E  =  30-40°.  f—  a.  =  0.020.  Both  are  difficultly 
soluble  in  hot  hydrochloric  acid.  Chrome  ocher  appears  very  similar  and  is 
likewise  brilliant  green.  It  is  nearly  uniaxial,  negative,  a  yellowish  to 
colorless,  B  =  c  brilliant  green.  It  is  found  particularly  as  a  halo  around 
chrome  spinel  in  serpentine. 

It  is  not  determined  whether  the  small  greenish-yellow,  radial,  fibrous 
spheres  with  a  perfect  cleavage  that  occur  in  quartz  schists  and  alum  schists 
and  are  called  astrolite,  belong  here  or  not.  Aside  from  a  lower  double  re- 
fraction, they  possess  properties  very  similar  to  mica.  The  number  of 
names,  which  micaceous  aggregates  have  received,  is  so  large  that  they 
cannot  be  discussed  in  this  text. 


310  PETROGRAPHIC  METHODS 

Chlorite  Group  and  Serpentine  (14) 

The  minerals  of  the  chlorite  group  in  the  narrower  sense  can 
but  rarely  be  recognized  macroscopically  as  rock  constituents  and 
are  generally  dark  green.  Perfect  micaceous  cleavage,  low 
hardness  and  flexibility  of  the  flakes  are  characteristic.  They 
are  generally  so  fine  scaly  that  the  individual  flakes  cannot  be 
recognized  distinctly.  They  have  a  somewhat  parallel  arrange- 
ment giving  to  the  rock  a  soft  velvety  luster.  The  normal 
chlorites,  i.e.,  those  members  of  the  series  rich  in  iron,  together 
with  sedgy  hornblende,  constitute  the  most  frequent  green  pig- 
ments of  crystalline  rocks,  and  numerous  greenstones,  green- 
schists,  etc.,  owe  their  color  to  it.  Leuchtenbergite,  which  is  free 
from  iron  and  colorless,  is  very  rare  and  is  scarcely  ever  visible 
macroscopically  while  the  dark  red  to  violet  members  containing 
chromium,  Cotschubeyite  and  cdmmererite  or  rhodochrome  are  con- 
fined to  secondary  formations  in  serpentines  rich  in  chromite. 

Distinction  is  made  under  the  microscope  between  pennine, 
which  is  sometimes  positive  and  sometimes  negative,  uniaxial 
with  parallel  extinction  and  frequently  shows  anomalous  lavender 
blue  to  rust  brown  interference  colors,  and  clinochlore.  The 
latter  has  higher  double  refraction,  is  biaxial,  and  always  positive. 
It  shows  normal  interference  colors  and  on  account  of  its  not 
insignificant  oblique  extinction  it  often  shows  twinning  lamina- 
tion according  to  the  mica  law  in  thin  section.  These  minerals 
are  very  light  in  thin  section,  but  are  generally  distinctly  colored 
and,  whether  they  are  positive  or  negative,  they  show  stronger 
absorption  of  the  rays  vibrating  in  the  cleavage  plane.  Colorless 
cross  sections  of  a  mineral  with  properties  similar  to  chlorite 
and  often  with  quite  anomalous  interference  colors  appear 
occasionally  in  the  rocks  chiefly  as  a  decomposition  product  of 
silicates  poor  in  iron.  It  has  been  identified  as  leuchtenbergite. 
Cotschubeyite  corresponds  to  clinochlore  in  its  optical  behavior 
while  cammererite  frequently  has  anomalous  interference  colors 
and  is  similar  to  pennine.  Both  are  hyacinth  red  parallel  to 
c  =  c  and  perpendicular  to  c  deep  violet.  Deep  blue  chlorite, 
erinite,  has  been  observed  as  a  rock  constituent.  It  is  yellow- 
ish parallel  to  c  =  a  and  cobalt  blue  perpendicular  to  c. 

Pleochroic  halos  with  brown  colors  are  common  in  chlorites. 
Scarcely  a  trace  of  an  interference  figure  can  be  observed  in  cleav- 
age plates  on  account  of  the  deep  color  and  the  low  double 
refraction.  When  roasted  in  air  the  chlorites  become  deep  brown 


DESCRIPTIVE  SECTION 


311 


(010) 


and  show  great  differences  in  absorption,  but  they  fuse  with 
difficulty.  They  can  be  etched  out  of  rocks  with  hydrochloric 
acid,  but  leave  gelatinous  silica  behind. 

Coarse,  scaly  aggregates  of  chlorite  are  found  as  secondary 
veins  in  serpentine  and  as  a  facies  of  that  rock.  These  aggregates 
frequently  contain  large  crystals  of  magnetite  and  occur  as  irregu- 
lar non-schistose  masses.  They  consist  principally  of  anomalous 
pennine  which  often  has  a  radial  flaky  ap- 
pearance, Fig.  325.  The  same  species  is 
also  common  as  an  alteration  product  of 
basic  silicates  and  sometimes  forms  pseu- 
domorphs  after  biotite.  These  are  always 
penetrated  by  titanium  minerals  and  have 
abundant  pleochroic  halos.  Sometimes  it 
forms  irregular  scaly  aggregates,  which 
have  developed  from  hornblende  or 
pyroxene;  also  from  almandine,  feldspar, 

etc.  The  same  formation  is  found  in  greenstones  and  propyl- 
lites  as  'a  fine  pigment  throughout  the  whole  rock.  It  is  not 
definable  optically  and  is  called  viridite.  In  piezocrystalline 
rocks,  especially  in  the  central  granites,  large  poorly  developed 
flakes  of  pennine  are  undoubtedly  primary  constituents.  These 
often  grow  parallel  with  biotite,  but  are  distinguished  from 
secondary  chlorites  by  a  lack  of  by-products  and  by  the  perfect 
clearness  of  the  mica. 


(001) 


FIG.  325. 
Pennine  Cleavage  Plate. 


ffi, 


FIG.  327. 


Clinochlore, 


Parallel  Cleavage. 


Parallel  Plane  of  Symmetry. 


The  tabular  individuals  of  chlorite  occurring  in  green  contact 
rocks  of  all  kinds,  amphibolite,  eclogite,  green  schist  and  chlorite 
schist,  belong  chiefly  to  clinochlore,  Figs.  326  and  327.  It  shows 
twinning  lamination  and  normal  interference  colors.  In  denser 


312  PETROGRAPHIC  METHODS 

formations  approaching  phyllites,  a  mineral  similar  to  chlorite 
occurs  but  is  distinguished  from  clinochlore  by  higher  indices  of 
refraction  but  they  do  not  reach  those  of  the  brittle  micas  al- 
though they  approach  them. 

The  table,  page  270,  shows  the  distinction  between  the  chlorites  and  other 
micaceous  minerals.  A  series  of  dark  green  minerals,  generally  with  low 
but  sometimes  with  high  double  refraction,  f—  a  =  0.02,  is  appended  to 
the  chlorites.  They  are  called  delessite,  ripidolite,  etc.,  and  some  of  them 
at  least  are  chlorites  rich  in  iron.  Likewise  silicates  very  rich  in  iron  with 
the  general  habit  of  chlorite  are  found  frequently  as  predominant  constit- 
uents of  sediments  particularly  of  oolite.  They  are  called  thuringite, 
owenite,  chamosite,  etc.,  and  form  scaly  cleavable  aggregates  with  a  specific 
gravity  of  about  3 . 2.  They  are  similar  to  chlorite  in  all  the  optical  proper- 
ties, being  negative  with  a  small  optic  angle  and  having  a  positive  principal 
zone. 

The  relations  between  the  chlorite  minerals  and  serpentine 
have  as  yet  not  been  thoroughly  determined.  There  is  no  doubt 
that  flaky  serpentine  or  antigorite  is  quite  analogous  to  the 
chlorites  in  its  optical  behavior  and  the  chemical  composition  of 
chlorite  rich  in  magnesium  has  been  explained  by  an  isomorphous 
mixture  of  magnesium  aluminium  silicate  called  amesite  with 
magnesium  silicate,  serpentine.  Besides  the  serpentine  minerals, 
which  belong  here,  there  is  a  fibrous  serpentine  or  chrysotile,  that 
has  no  relation  whatever  to  the  chlorite  group. 

The  serpentine  minerals  are  not  known  in  individual  crystals. 
All  occurrences  so  designated  are  pseudomorphs.  Antigorite  in 
deep  green,  flaky  masses  and  chrysotile  in  yellowish-green,  fine, 
fibrous  masses  with  a  silky  luster  and  fine  enough  to  spin,  are 
observed  in  crevices  of  serpentine  consisting  for  the  most  part  of 
very  fine  aggregates  of  that  mineral.  The  principal  field  of 
distribution  of  the  mineral  is  in  those  rocks,  which  are  macro- 
scopically  compact  with  a  fine  splintery  fracture  and  streaked 
with  dark  green  or  red,  rarely  with  honey  yellow. 

The  appearance  of  the  two  varieties  of  serpentine  under  the 
microscope  is  quite  variable  in  spite  of  the  similarity  with  respect 
to  index  and  double  refraction.  Both  are  generally  colorless  in 
thin  section  and  are  therefore  not  pleochroic.  If  pleochroic 
varieties  are  thought  to  have  been  found,  mllarsite,  chemical 
observation  will  not  confirm  it  and  the  mineral  is  undoubtedly 
chlorite,  which  is  so  often  mixed  with  serpentine.  Figs.  328  and 
329  show  the  optical  constants  of  the  two  types  of  serpentine  and 
in  these  the  difference  in  habit  appears  distinctly.  However,  the 


DESCRIPTIVE  SECTION 


313 


A  e=^  B 


\ 

/ 

/ 

\ 

\ 

r 

/f 

/ 

\ 

/ 

\ 

/ 

\ 

/ 

\ 

M                                       B    /r  A. 

FIG.  328.                                         FIG.  329. 

question  is  by  no  means  clear  in  all  respects  and  there  are  un- 
doubtedly forms  of  development  in  genuine  serpentine,  which 
could  not  be  classified  under  either  of  the  diagrams. 
•  New  names  cannot  be  given  to  these  dense  aggregates  simply 
because  they  have  different  habit  as  has  been  done  in  the  anti- 
gorite  varieties,  picrosmine,  metaxite,  picrolite,  etc.,  and  likewise 
the  microscopic  forms  of  aggregation,  e.g.,  the  radial,  which  has 
lately  given  rise  to  the  name  radiotine,  cannot 
be  recognized  as  sufficient  ground  for  evolv- 
ing a  new  name. 

Serpentine  is  undoubtedly  very  rare  as  a 
primary  constituent  of  rocks.  Antigorite  as 
such  has  been  men- 
tioned and  shown 
in  Fig.  288,  page 
276.  It  occurs  in 
parallel  growth 
with  olivine.  If  an 
olivine  rock  is  in- 
tergrown  with 
antigorite,  which 
represents  a  typi- 
cal formation  of 

piezocrystallization,  stubachite,  later  suffers  more  or. less  com- 
plete serpentinization  by  thermal  processes,  a  very  fine,  scaly, 
irregular  aggregate  of  secondary  serpentine  is  developed 
between  the  lath-shaped  crystals  of  the  primary  antigorite. 
The  secondary  serpentine  is  dense  and  compact  and  scarcely 
acts  upon  polarized  light  while  the  laths  of  fresh  antigorite 
appear  like  a  lattice  in  it,  lattice  structure,  Fig.  330,  and  often 
fine,  granular  residues  of  olivine  that  are  scarcely  transparent 
are  seen.  This  development  is  to  be  sure  very  typical,  but  by 
no  means  constant.  It  gives  way  to  radial  fibrous  and  con- 
fused, almost  structureless  masses.  The  various  antigorite 
serpentines  are  characteristic  for  regions  in  which  the  rocks 
have  been  formed  under  the  influence  of  piezocrystallization. 
They  are  predominant  under  such  circumstances,  but  are  excep- 
tional under  normal  conditions. 

The  appearance  of  crysotile  serpentine  is  unlike  that  of  anti- 
gorite. It  forms  fibrous  masses  perpendicular  to  the  edges  and 
filling  out  the  cracks  of  olivine  thus  causing  the  mineral  to  crack 


Optical  Orientation  of 
Antigorite.  Chrysotile. 


314  PETROGRAPHIC  METHODS 

further  and  these  cracks  are  again  filled  with  fine  fibrous  aggre- 
gates. So  the  process  goes  on  until  the  characteristic  appearance 
of  the  mesh  structure  is  seen,  Fig.  331.  Small  grains  of  more  or 
less  clear  olivine  may  be  present  in  it  or  the  whole  may  be  altered 
to  serpentine.  In  the  veins  of  such  fibrous  serpentine  there 
are  generally  two  differents  kinds  of  substances,  which  appear 
quite  similar.  On  the  outer  border  the  axes  of  the  fibers  are 
positive  and  the  middle  portion  is  also  fibrous  with  the  crystals 
perpendicular  to  the  sides  of  the  vein,  but  the  principal  zone  is 
negative. 

Serpentine  as  a  rock  is  always  developed  from  peridotites. 
Other  rocks,  e.g.,  pyroxenite  and  amphibolite,  never  turn  to 
serpentine.  It  is  always  a  product  of  thermal  activity,  see 


FIQ.  330. — Serpentine  with  Lattice  Struc-       FIG.  331. — Serpentine  with  Mesh  Structure, 
ture.     Hackbrettl  in  Stubachtal.  Trogen  near  Hof,  Fichtelgebirge. 

Allgemeine  Gesteinskunde,  page  152.  Now  and  then  pyroxenes 
low  in  aluminium  in  such  rocks  are  effected  by  this  process  of 
alteration  and  a  pseudomorph  of  antigorite  is  formed  from  it, 
bastite.  Generally,  however,  these  minerals  remain  unchanged 
and  in  the  serpentines  of  the  Central  Alps  a  few  individuals  of 
pyroxene  constitute  the  only  residue  of  the  original  rock  and  this 
gave  rise  to  the  opinion  that  the  whole  rock  was  an  altered 
pyroxenite — pyroxene  serpentine.  By-products  are  very  wide- 
spread in  the  formation  of  serpentine  from  olivine.  Besides  iron 
ores,  which  always  occur,  chlorite,  talc  and  actinolite  are  formed. 
The  latter  material  has  been  falsely  considered  as  the  parent 
substance  of  serpentine, — hornblende  serpentine. 


DESCRIPTIVE  SECTION  315 

The  pseudomorph  formations  are  also  formed  in  rocks  in  which 
olivine  is  a  subordinate  constituent  as  in  the  contact-metamor- 
phosed carbonate  rocks  where  the  serpentine  is  generally  poor  in 
iron  and  is  yellowish-green  to  sulphur  yellow — precious  serpen- 
tine— but  it  is  sometimes  dark  green  to  pure  black.  The  latter 
color  is  found  especially  in  certain  ophicalcites  called  eozoon. 
Besides  forsterite,  humite  may  be  the  parent  material  in  such 
rocks  and  in  both  cases  the  serpentine  shows  the  typical  mesh 
structure.  More  rarely  it  is  formed  from  periclase  and  the 
cubical  cleavage  shows  even  macroscopically.  In  the  pseudo- 
morphs  the  antigorite  flakes  lie  parallel  to  the  cube  faces  and 
overlap  each  other  in  such  a  manner  that  no  double  refraction 
can  be  observed  in  thin  section.  Fresh  fragments  of  the  original 
mineral  are  sometimes  seen  as  small  rounded  inclusions  in  calcite 
grains. 

Sometimes  pseudomorphs  after  an  olivine  rich  in  iron  are 
found  in  olivine-bearing  porphyric  plagioclase  rocks.  These  have 
a  cleavage  like  that  of  mica  and  are  similar  to  bastite.  They  are 
distinctly  green  in  thin  section  and  pleochroic,  c  green,  B  and  a 
yellow.  They  also  have  quite  a  strong  double  refraction, 
f—  a  =  0.025.  They  are  serpentine  very  rich  in  iron.  The 
color  and  pleo^chroism  become  more  intense  upon  oxidation  and 
when  such  pseudomorphs  become  brown  they  show  very  signifi- 
cant difference  in  absorption  from  brownish-red  to  yellowish 
c>fi>a,  iddingsite  or  bowlingite.  Similar  substances  are  found 
in  the  same  rock  occurring  principally  with  the  mesh  structure 
and  when  they  are  brown  the  serpentines  rich  in  iron  can  be 
made  deeper  brown  and  strongly  pleochroic  by  roasting  in  the 
air.  The  common  serpentines  remain  unchanged  by  this  treat- 
ment so  long  as  they  do  not  lose  their  water.  Serpentine  can  be 
distinguished  from  chlorite,  which  appears  very  similar  to  it,  by 
roasting  with  cobalt  solution  when  the  latter  becomes  blue. 
Sometimes  it  is  necessary  to  dissolve  the  serpentine  out  with 
hydrochloric  acid  in  order  to  expose  accessory  minerals  concealed 
in  it. 

Black  serpentine  mentioned  above  must  be  briefly  described  here.  It 
occurs  in  many  ophicalcites.  In  thin  section,  it  generally  forms  a  mass 
like  graphite,  but  a  few  individuals  are  prominent  from  their  larger  dimen- 
sions. They  show  complete  absorption  even  in  the  thinnest  sections  parallel 
to  the  principal  zone  and  perpendicular  to  it  they  are  completely  colorless. 
Hydrosilicates  containing  nickel  belong  in  this  group.  They  occur  as  a 


316 


PETROGRAPHIC  METHODS 


secondary  development  in  crevices  in  serpentine  and  represent  one  of  the 
most  important  nickel  ores,  numeite,  genthite,  pimelite,  etc.  They  are 
fibrous  scaly  substances  with  brilliant  green  to  yellowish-green  color,  sp. 
gr.  2.6-2.9.  Optically  positive  with  medium  double  refraction  and 
positive  principal  zone.  They  turn  black  before  the  blowpipe  and  are 
attacked  by  acids  with  difficulty. 

Hydrargillite  (14) 

Hydrargillite  is  formed  occasionally  in  the  decomposition  of  various 
feldspars  especially  in  the  formation  of  bauxite.  It  has  also  been  found  in 
spreustein  and  a  few  occurrences  of  emery.  The  aggregates  appear  fibrous 
in  thin  section  and  are  extremely  difficult  to  determine  because  various 
occurrences  have  different  optical  properties.  Strong  double  refraction, 
negative  principal  zone  and  oblique  extinction  are  characteristic  and  dis- 
tinguish the  mineral  from  brucite,  kaolin,  talc,  muscovite,  leuchtenbergite, 
etc.,  which  appear  similar  to  it.  Diaspore  occurs  with  it  and  is  recognized 
by  much  higher  indices  of  refraction.  Chemical  reactions,  such  as  the  blue 
color  upon  roasting  with  cobalt  solution  and  the  lack  of  silica,  are  sometimes 
employed  to  determine  the  diaspore. 


Fio.  332.— Hydrargillite, 
Cleavage  Plate. 


FIG.  333. — Talc,  Cleavage  Plate. 


Talc  (15) 


Talc  as  a  rock-forming  mineral  has,  by  no  means,  the  signifi- 
cance which  geology  in  general  ascribes  to  it.  Most  of  the 
occurrences  so  designated  are  scaly  aggregates  of  sericite  as 
mentioned  above.  Talc  has  a  much  more  greasy  feel,  but  the 
two  cannot  be  differentiated  under  the  microscope.  Chemical 
investigation  alone  furnishes  a  clue  for  the  distinction  and  this 
may  be  either  the  hydrofluosilicic  acid  reaction,  page  163,  or 
roasting  the  mineral  with  cobalt  solution,  when  sericite  becomes 
brilliant  blue. 

Talc  rocks  in  general  consist  predominantly  of  fine,  confused, 
scaly  aggregates  of  talc,  such  as  talc  schists,  soapstone,  potstone, 


DESCRIPTIVE  SECTION 


317 


etc.  They  are  very  soft  and  can  be  carved  or  turned,  but  upon 
roasting  they  attain  a  hardness  greater  than  that  of  quartz. 
They  are  always  anomalous  formations,  which  have  resulted 
from  intense  chemical  processes  sometimes  in  the  neighborbood 
of  the  most  acid  eruptive  rocks,  the  granites,  and  sometimes  they 
have  formed  from  the  most  basic  rocks,  the  peridotites,  in  which 
frequently  the  whole  rock  is  changed  to  talc  without  any  residue. 
Rhombohedrons  of  magnesite,  prisms  of  actinolite  and  micro- 
lites  of  rutile  are  frequently  by-products  occurring  in  the  yellow- 
ish, grayish,  or  greenish  aggregates  of  talc.  Talc  is  found  occa- 
sionally as  a  subordinate  constituent  in  occurrences  that  scarcely 
deserve  the  name  of  rock,  such  as  listwanite  and  duelho.  It  also 
occurs  in  magnesite  and  numerous  serpentines. 

Pyrophyllite  (15) 

The  distribution  of  pyrophyllite  as  a  rock  constituent  cannot  be  estimated 
because  it  is  very  difficult  to  distinguish  it  from  muscovite  in  thin  section 
except  that  it  has  a  larger  optic  angle.  In  cases  of  doubt  a  decision  can  be 
reached  by  means  of  the  hydrofluosilicic  acid  reaction.  It  is  sometimes 
formed  as  a  by-product  in  the  process  of  kaolinization  and  other  similar 


X^54r 


(001) 


(010) 


/ 


\ 


FIG.  335. 


Pyrophyllite, 
Cleavage  Plate.  Macropinacoid. 

replacement  processes.  It  occurs  in  isolated  scales  and  rarely  in  dense 
aggregates  of  agalmatolite  similar  to  soapstone.  When  roasted  with  cobalt 
solution  they  become  blue.  It  is  more  frequently  found  as  a  fine,  scaly 
coating  on  clay  slates  in  which  it  often  occurs  in  silvery  aggregates  covering 
crevices  or  as  an  agent  of  petrifaction,  of  grapholites,  gumbelite,  for  example. 

Bertrandite  (15) 

Limpid,  tabular  crystals  of  bertrandite  have  been  found  in  certain  peg- 
matites. It  occurs  also  as  a  constituent  of  granites,  aplites,  etc.,  but  is 
very  difficult  to  determine  on  account  of  its  great  similarity  to  muscovite. 


318 


PETROGRAPHIC  METHODS 


It  is  very  difficult  to  find  in  any  case  because  its  specific  gravity  is  about 
the  same  as  that  of  the  principal  constituents  occurring  in  the  rocks  and 
it  cannot  be  separated.  The  hydrofluosilicic  acid  reaction  is  of  value  to 
distinguish  it  from  muscovite. 

Wagnerite  (15) 

Wagnerite  is  probably  much  more  widespread  than  it  is  at  present 
supposed  to  be.  It  is  only  known  as  an  associate  of  siderite  in  phyllitic 
rocks.  Its  optical  properties  are  not  very  characteristic,  but  it  may  be 
isolated  quite  easily  on  account  of  its  high  specific  gravity.  The  reaction 
for  phosphate  is  a  good  test  for  it. 

Kaolin  (15) 

Feldspar  rocks  alter  to  kaolin  by  post-volcanic  processes  and 
the  alteration  affects  the  plagioclase  first  and  later  the  ortho- 
clase,  while  imcrocline  resists  it  almost  entirely.  Kaolin  forms 
extremely  fine,  scaly  aggregates,  which  so  far  as  is  positively 


FIG.  337. 


Cleavage  Plate. 


Kaolin, 


Plane  of  Symmetry. 


known,  are  always  porous  and  are  scarcely  ever  observed  in  thin 
section.  The  usual  alteration  products  of  feldspar,  which  cause 
the  cloudiness  that  is  so  widespread  in  them,  do  not  belong  to 
kaolin  as  is  commonly  supposed,  but  are  distinguished  from  it 
even  in  the  densest  aggregates  by  a  much  more  brilliant  inter- 
ference color.  It  is  probably  sericite. 

When  the  presence  of  kaolin  can  be  positively  proved,  the 
rocks  are  so  much  altered  that  the  kaolin,  which  is  easily  sus- 
pended in  water,  can  be  isolated  by  simply  washing  and  it  can 
be  identified  by  chemical  tests.  The  mineral  is  quite  important 
as  a  constituent  of  secondary  deposits  of  kaolin  sandstone  and 


DESCRIPTIVE  SECTION  319 

kaolin  clay.  It  is  by  no  means  positively  determined  to  what 
extent  it  occurs  in  plastic  and  non-plastic  clays  because  the  clay 
substance,  that  is  always  present  in  these  sediments  and  is 
called  kaolin,  in  many  cases,  does  not  prove  to  be  kaolin  by 
chemical  analysis.  The  same  is  true  for  its  distribution  in  the 
normal  products  of  weathering  and  in  soils,  in  which  kaolin  was 
assumed  as  a  constituent,  without  further  investigation.  It  is 
doubtful  whether  the  scaly  mineral  in  such  occurrences  can  be 
distinguished  from  kaolin  or  not,  although  it  differs  from  kaolin 
greatly  in  chemical  composition,  being  an  alkali  mineral.  Differ- 
entiation may  be  accomplished  by  an  application  of  the  method 
of  Schroeder  van  der  Kolk,  Part  I,  page  38. 

Nontronite  (15) 

Nontronite  is  not  a  rare  constituent  of  greatly  altered  eruptive  and  con- 
tact rocks.  It  may  be  formed  from  any  mineral  whether  it  bears  ferric 
oxide  or  not,  and  occurs  especially  in  rocks  that  have  been  so  shattered  that 
the  inner  texture  is  entirely  porous.  It  sometimes  forms  a  scaly  sulphur 
yellow  coating  in  crevices  of  rocks  or  it  penetrates  the  whole  rock  in  a 
regular  manner  and  forms  earthy  pseudomorphs  after  it.  Greenish-yellow 
masses  with  a  choncoidal  fracture  and  impregnated  with  opal,  chloropal, 
are  not  rare  in  such  cases.  Nontronite  is  nearly  always  determined  as 
epidote  on  account  of  its  color  and  double  refraction  but  is  distinguished 
from  it  by  much  lower  indices  of  refraction. 

A  mineral  very  similar  to  nontronite  in  appearance  and  optical  properties 
is  very  common  as  a  crust  in  crevices  especially  on  the  surface  of  rocks  con- 
taining pyrite.  This  is  copiapite,  a  basic  ferrous  sulphate,  that  is  decom- 
posed in  water. 

Hydromagnesite  (15) 

Hydromagnesite  is  found  in  dense  masses  as  pseudomorphs  after  minerals 
rich  in  magnesium  and  also  in  flaky  or  fibrous  aggregates  in  crevices  of  rocks 
rich  in  magnesium.  It  generally  shows  twinning  lamination  similar  to 
that  of  plagioclase.  The  aggregates  are  very  fine,  fibrous  and  irregular 
and  can  be  recognized  with  difficulty.  Effervescence  shows  its  presence  in 
a  thin  section  if  other  carbonates  are  not  present. 

Cordierite  (15) 

Cordierite  forms  crystals  with  short  prismatic  habit  that  are 
usually  quite  distinctly  visible  macroscopically  in  granites, 
quartz  porphyries  and  other  extrusive  rocks,  but  become  micro- 
lites  in  fritted  sandstones.  They  are  frequently  in  the  form  of 


320 


PETROGRAPHIC  METHODS 


(01O) 


(010) 


FIG.  338. — Cordierite,  Section 
through  a  Trilling. 


penetration  twins  appearing  in  cross  section  as  shown  in  Fig. 
338.  The  edges  are  generally  rounded  and  the  crystals  corroded 
in  varied  manners.  It  is  also  found  quite  widespread  in  the 
form  of  grains  in  clay  slate  hornfels,  and  these  often  show  de- 
cided twinning  lamination.  A  portion  of  the  knotty  aggregates 
in  a  spotted  slate,  Knotenschiefer,  consists  of  poorly  developed 
individuals  of  cordierite  very  rich  in  in- 
clusions. In  these  rocks  it  is  colorless  in 
thin  section  and  is  frequently  penetrated 
by  long  slender  needles  of  sillimanite, 
Fig.  212,  page  196.  It  also  contains  a 
large  number  of  inclusions  of  rutile  and 
zircon  with  typical  yellow  pleochroic 
halos  in  which  the  double  refraction  is 
lower,  but  the  dispersion  of  the  optic 
axes  and  the  indices  of  refraction  are 
higher  and  the  ray  vibrating  parallel  to  c  appears  brilliant  yellow. 
If  such  characteristic  inclusions  are  lacking  in  colorless  cor- 
dierite it  is  distinguished  from  quartz  by  observation  in  con- 
vergent light,  by  special  chemical  reactions,  page  174,  and  by 
the  beginning  alteration  of  cordierite  into  greenish  micaceous 
aggregates,  which  begins  on  the  borders  or  at  the  inclusions. 
The  pseudomorphs  sometimes  consist  of  parallel  aggregates, 
gigantolite,  and  sometimes  of  irregular  masses  similar  to  snow 
crystals  in  polarized  light,  pinite.  The  pleochroic  halos  often 
remain  as  brown  specks  after  the  alteration.  Cordierite  often 
appears  very  similar  to  plagioclase  on  account  of  its  twinning 
lamination.  If  the  characteristics  mentioned  above  are  lacking, 
its  determination  in  thin  section  may  become  very  difficult.  The 
hydrofluosilicic  acid  reaction  affords  positive  proof  of  the  mineral. 
When  it  occurs  as  a  fresh  constituent  of  extrusive  rocks  or  in 
tuffs  it  is  generally  decidedly  blue  in  thin  section  and  then  shows 
distinctive  pleochroism.  It  becomes  nontransparent  before  the 
blowpipe  and  melts  with  difficulty. 

Wavellite  (15) 

Wavellite  is  not  rare  in  crevices  of  sedimentary  rocks  in  which  it  forms 
perfectly  radial  aggregates.  It  has  apparently  resulted  from  decom- 
position of  organic  phosphates.  Its  distribution  has  never  been  deter- 
mined microscopically,  but  the  extraordinarily  typical  form  of  aggregation 
together  with  the  high  double  refraction  afford  some  clues  to  the  identity 
of  the  mineral. 


DESCRIPTIVE  SECTION 


321 


B ---J 


Gypsum  (15) 

Gypsum  is  a  prominent  constituent  of  numerous  members  of 
the  halite  formations,  but  it  is  also  found  in  sedimentary  rocks 
other  than  these.  Its  origin  from  anhydrite  can  be  determined 
in  many  cases.  It  is  also  found  as  a  product  of  solfataras  and 
sulphur  springs  and  is  sometimes  formed  from  carbonate  rocks  or 
occurs  as  impregnations  in  volcanic  tuffs.  In  the  latter  case 
it  is  generally  accompanied  by  sulphur. 
Gypsum  is  often  found  as  a  secondary 
formation  in  the  iron  caps  of  sulphide  ore 
deposits  and  in  calcareous  rocks  where 
sulphides  have  been  altered. 

Rocks  consisting  predominantly  of 
gypsum  are  very  soft  and  are  generally 
crossed  by  numerous  veins  of  fibrous 
gypsum.  The  color  is  white  or  gray,  due  (100) 
to  carbonaceous  substance,  or  yellow  to 
red,  due  to  ferric  hydroxide.  If  gypsum 
occurs  as  a  subordinate  constituent  of 
rocks  it  can  only  be  recognized  when 
cleavage  plates  with  pearly  luster  appear 
distinctly.  It  is  difficult  to  determine 
under  the  microscope.  Twinning  lamina- 
tion and  fibrous  fracture,  which  are  distinctly  visible  in  crystalline 
aggregates,  are  rarely  observed  in  the  denser  varieties.  Positive 
evidence  of  the  presence  of  gypsum  in  rocks  is  obtained  by  leach- 
ing the  rock  with  water  when  small  microscopic  crystals  of 
gypsum  are  formed  from  the  solution. 

Feldspar  Group  (16) 

The  feldspars  are  the  most  widespread  rock-forming  minerals. 
Accurate  investigation  of  them  is  especially  interesting  on  ac- 
count of  their  importance  because  they  form  the  basis  for  classi- 
fying the  eruptive  rocks.  Separation  of  monoclinic  orthoclase 
from  triclinic  plagioclase  is  far  from  sufficient  for  the  modern 
petrographical  classification.  It  is  also  recognized  that  a  natural 
petrographical  system  founded  on  a  chemical  basis,  is  not  possible 
without  determining  the  exact  properties  of  the  feldspar.  For 
these  reasons  many  investigators  have  set  about  to  determine 
the  composition  of  these  minerals  in  an  optical  way  without  the 
21 


FIG.  339. — Gypsum,  Plane 
of  Symmetry. 


322 


PETROGRAPHIC  METHODS 


aid  of  chemical  analyses,  which  at  best  require  much  time  and 
are  often  not  possible.  Many  methods,  giving  results  in  a  more 
or  less  direct  manner,  have  been  worked  out  so  that  now  an 
experienced  petrographer  can  determine  quite  accurately  with 
the  microscope,  the  chemical  properties  of  a  plagioclase  in  thin 
section. 

The  dimensions  of  rock-forming  feldspars  are  extremely 
variable.  Orthoclase  crystals  several  cubic  meters  in  size  occur 
in  certain  pegmatites,  and  crystals  from  the  size  of  an  egg  to  those 
the  size  of  a  man's  head  occur  in  granite  porphyries.  All  possible 
dimensions  are  found  from  these  down  to  the  minutest  microlites 
of  very  glassy  rocks.  The  microlites  are  often  so  small  that  even 
in  a  thin  section  several  individuals  may  overlap  each  other. 
The  plagioclases  develop  into  large  crystals  much  less  than 
orthoclase  and  in  most  instances  they  cannot  be  determined 
macroscopically. 

The  crystallographic  development  of  orthoclase  is  in  the  main 
not  very  different  from  that  of  plagioclase.  The  combination 
occurring  most  frequently  on  adularia  crystals,  viz.,  a  prism  with 


FIG.  340. 


FIG.  342. 


Usual  Types  of  Feldspar  Crystals. 

a  rear  hemidome,  Fig.  340,  appears  to  be  the  typical  form  of 
an  orthoclase  and  the  cross  sections  are  principally  rhombic — 
rhombporphyry ,  Fig.  343.  They  are  sometimes  rounded  and 
sometimes  elongated  like  a  lancet.  Another  type  of  development 
particularly  common  on  orthoclase  shows  the  above  combination 
modified  by  the  addition  of  a  side  pinacoid  which  truncates  the 
sharp  edges  of  the  prism,  Fig.  341.  The  basal  pinacoid  and  a 
number  of  other  end  faces  are  often  present.  Fig.  178,  page  181, 
shows  the  appearance  of  cross  sections  of  the  more  isometric 
individuals.  The  side  pinacoid  may  become  more  and  more 
dominant  in  the  combination  and  the  crystals  assume  a  thick 
or  thin  tabular  habit.  This. is  seen  characteristically  in  cross 


DESCRIPTIVE  SECTION  323 

sections  of  plagioclase  rich  in  lime,  Fig.  344.  Finally  a  type 
elongated  parallel  to  the  a  axis  is  quite  widespread,  Fig.  342. 
Cross  sections  of  it  appear  quadratic  in  orthoclase  and  nearly 
quadratic  in  plagioclase,  about  93°,  Fig.  371,  page  338,  because 
of  the  equal  development  of  the  basal  and  side  pinacoids. 

The  cleavage  of  the  feldspars  is  about  the  same  in  the  different 
members,  but  in  thin  section  it  occurs  in  various  ways,  according 
to  the  condition  of  the  feldspar.  A  system  of  long  sharp  cracks, 
parallel  to  the  base  and  less  perfect  ones  parallel  to  the  side  pina- 
coid  are  seen  in  thin  sections  of  the  fresh  unaltered  occurrences 


FIG.  343. — Rhombporphyry.     Kolsaas  FIG.  344. — Labradorite  Porphyrite  with 

near  Cristiania.  Tabular  Plagioclase. 

of  the  Central  Alpine  granites  and  schists.  Parting  parallel  to 
the  orthopinacoid  often  appears  more  distinctly  in  fresh  sanidine. 
Cleavage  is  less  perfect  in  the  clouded  occurrences  and  is  often 
indicated  in  thin  section  more  by  the  arrangement  of  decomposi- 
tion products  than  by  real  cracks.  When  the  rock  is  crushed,  the 
feldspars  cleave  very  differently.  In  orthoclase  cleavage  plates 
parallel  to  the  base  predominate,  while  in  plagioclase  plates 
parallel  to  the  side  pinacoid  are  most  common  because  of  the 
lamellar  development  parallel  to  it.  Microscopic  investigation 
of  these  cleavage  plates  in  parallel  and  convergent  polarized 
light  affords  considerable  data  for  determining  the  feldspar. 

Alteration  is  extremely  common  and  those  feldspars,  that  are 
colorless  when  fresh,  assume  a  clouded  appearance  and  often  be- 
come deeply  colored.  In  thin  section  the  color  can  be  seen  to  be 
due  to  inclusions.  Orthoclase  usually  appears  with  deep  red 


324 


PETROGRAPHIC  METHODS 


and  brown  colors  while  plagioclase  is  usually  not  so  intensely 
colored  and  is  more  apt  to  be  yellow  or  greenish.  In  zonal 
crystals  the  alteration  often  begins  in  the  center  and  proceeds 
outward  along  the  cleavage  cracks  or  it  attacks  the  various  zones 
differently.  The  usual  cloudiness  is  produced  in  orthoclase  as 
well  as  in  plagioclase  by  fine,  scaly  aggregates  of  sericite.  Kaolin, 
on  the  other  hand,  is  entirely  independent  genetically  and  is 
wholly  a  local  alteration. 

Mechanical  structures  are  not  very  widespread  in  the  feld- 
spars. The  occurrence  of  fractured  feldspar  crystals  in  normal 
extrusive  rocks  is  characterized  as  protodase.  The  twinning 
lamellae  of  plagioclase  are  often  bent  by  orogenic  processes  while 
orthoclase  frequently  assumes '  a  lattice  structure  similar  to 
microcline.  Both  may  be  crushed  and  assume  the  cataclase 
structure. 

The  feldspars  are  divided  into  two  groups: 

a.  Alkali  feldspars;  orthoclase,  microcline,  anorthoclase. 

b.  Soda  lime  feldspars  or  plagioclase. 

a.  The  Alkali  Feldspars 

The  alkali  feldspars  occur  as  rock  constituents  in  three  forms 
and  two  of  these  are  monoclinic,  orthoclase  and  sanidine.  They 
are  distinguished  by  the  size  of  the  optic  angle  which  in  ortho- 


FIG.  346. 


Orthoclase  Cleavage  Plates. 
Parallel  Base.  Parallel  Plane  of  Symmetry. 

clase  is  very  large  and  in  sanidine  is  always  much  smaller  and  is 
often  zero.  Further,  the  plane  of  the  optic  axes  in  orthoclase  is 
always  perpendicular  to  the  plane  of  symmetry  and  in  sanidine  it 


DESCRIPTIVE  SECTION 


325 


is  sometimes  parallel,  Figs.  345  to  348.  In  the  last  case,  strong 
inclined  dispersion  of  the  optic  axes  is  especially  noticeable  and 
this  is  characteristic  for  sanidine.  When  orthoclase  is  slowly 
heated,  it  may  be  noted  that  the  optic  angle  gradually  diminishes 
to  zero  and  at  about  500°  the  axes  separate  again  in  a  plane  per- 
pendicular to  the  first.  When  heated  for  a  long  time  at  a  tem- 
perature above  600°  the  orientation  remains  the  same  even  upon 
cooling.  The  geological  conclusions  concerning  the  temperature 


FIG.  348. 


Sanidine,  Cleavage  Plates, 
Parallel  Base.  Parallel  Plane  of  Symmetry. 

at  which  the  rocks  are  formed,  drawn  from  this  phenomenon, 
have  been  shown  to  be  entirely  intenable.  Triclinic  microcline 
is  the  third  form  of  alkali  feldspar. 

Orthoclase  sometimes  develops  in  well  outlined  crystals  which 
are  generally  distinctly  visible  macroscopically  and  may  attain 
considerable  size,  especially  in  granite  porphyry.  Sometimes  the 
crystals  are  somewhat  rounded.  The  small  microlitic  individuals 
in  the  ground  mass  of  trachytes,  etc.,  may  have  perfect  crystal- 
lographic  outline  especially  in  rocks  containing  glass.  The 
crystals  show  the  variable  habits  sketched  above.  The  cross 
sections  are  six-sided  or  rectangular  to  long  lath-shaped.  The 
last  form  is  common,  especially  in  the  minute  individuals  in  the 
ground  mass  of  trachytes,  etc.  Fig.  183,  p.  184,  shows  the  lath- 
shaped  cross  sections  of  this  type  with  fluidal  arrangement  and 
the  brittleness  of  sanidine  phenocrysts  also  appears  distinctly  in 
this  figure. 

Twins  are  extremely  widespread  and  generally  two  individuals 
lie  side  by  side  separated  by  a  more  or  less  straight  line.  Twins 


326 


PETROGRAPHIC  METHODS 


(001) 


(QOl) 


(100) 


(100) 


FIG.  349.— Carlsbad  Twin,  Section 
Parallel  Clinopinacoid. 


according  to  the  Carlsbad  law  are  most  common.     The  ortho- 
pinacoid  is  the  twinning  plane  and  the  development  may  be  such 

that  this  is  also  the  composition  plane, 
Fig.  349,  or,  as  is  more  frequently  the 
case,  the  two  individuals  are  grown 
together  with  the  clinopinacoid  for 
the  composition  plane,  Fig.  350.  The 
Baveno  is  another,  less  common 
twinning  law.  Here  the  clinodome 
{011}  is  the  twinning  plane.  This 
type  is  found  particularly  in  the  crys- 
tals developed  prismatic  along  the  a 
axis.  Fig.  351  shows  a  cross  section 
of  it.  The  two  individuals  are  almost 
exactly  at  90°  to  each  other  and 
therefore  extinguish  simultaneously, 
but  equivalent  vibration  directions 
are  crossed  in  them.  Twinning  parallel  to  the  base 
and  to  other  forms  occur  now  and  then  but  are  gener- 
ally very  difficult  to  determine.  Frequently  the 
crystals  show  zonal  development.  Zonal  arrange- 
ment of  inclusions  is  often  observed  in  fresh  indi- 
viduals. The  zonal  structure  appears  more  distinctly 
when  the  crystals  are  weathered  because  the  different 
zones  alter  differently. 

Regular  intergrowth  of  orthoclase  with  other  min- 
erals  is   frequent,   above  all  with  plagioclase.      The 

latter  may  penetrate  orthoclase 
throughout  in  parallel  position  so 
that  a  fine  network  of  acid  plagioclase 
with  orthoclase  results — perthite — Fig. 
352,  or  the  reverse  may  be  true — 
antiperthite.  A  section  perpendicular 
to  that  shown  in  Fig.  352  appears 
wholly  fibrous  even  in  ordinary  light. 
In  other  cases  a  few  irregular  elon- 
gated spindles  of  plagioclase  are  reg- 
ularly intergrown  with  orthoclase. 
Such  intergrowths  are  widespread  in  purely  microscopic  dimen- 
sions— microperthite — and  finally  they  become  so  fine  that  their 
presence  can  be  recognized  only  by  the  imperfect  extinction  of 


(001) 


(001) 


(010) 


(010) 

FIG.  351. — Baveno  Twin,  Section 
Parallel  Orthopinacoid. 


DESCRIPTIVE  SECTION  327 

the  cross  sections — cryptoperthite.  Orthoclase  sometimes  occurs 
as  a  border  around  plagioclase  crystals,  but  the  reverse  of  this  is 
a  rare  occurence. 

Graphic  intergrowth  of  orthoclase  and  quartz  is  very  impor- 
tant. Long  parallel  rods  of  quartz  penetrate  through  the  feld- 
spar crystals  and  the  cross  sections  of  the  quartz  show  an  angular 
outline  similar  to  writing,  Fig.  353.  This  intergrowth  is  very 
frequent  in  aplites  and  in  the  ground  mass  of  quartz  porphyry, 
but  often  in  microscopic  dimensions.  Then  it  is  called  micropeg- 
matite.  In  many  cases  this  becomes  more  and  more  indistinct. 


FIG.  352. — Perthitic   Intergrowth  of  FIG.  353. — Micropegmatite.     Aplite,  Col 

Orthoclase  and  Plagioclase.  de  Tourmalet,  Pyrenees. 

It  forms  an  extremely  fine  aggregate  often  appearing  somewhat 
radial — granophyre — until  finally  it  passes  over  into  a  dense 
aggregate  of  particles  that  are  only  transparent  in  the  thinnest 
sections  and  very  little  or  no  effect  at  all  can  be  observed  on 
polarized  light — microfelsite. 

In  another  kind  of  intergrowth  of  quartz  with  feldspar  the 
cross  sections  of  the  quartz  rods  are  rounded  and  worm-like — 
myrmicoidal  structure,  Fig.  259,  p.  248.  This  is  beautifully  seen 
in  rounded  grains  in  granite,  granulite,  and  quartz  diorite  and  in 
injected  schists  and  it  can  frequently  be  positively  proved  that 
in  this  intergrowth  the  feldspar  is  a  plagioclase. 

Inclusions  are  not  very  common  in  orthoclase,  but  all  of  the 
other  rock  constituents  may  be  found  as  inclusions  in  it.  The 
phenocrysts  of  a  granite  porphyry  may  be  quite  rich  in  such 
foreign  minerals.  Biotite  flakes  and  quartz  crystals  can  be  seen 
with  the  naked  eye  and  these  sometimes  show  the  zonal  arrange- 


328 


PETROGRAPHIC  METHODS 


ment.  Liquid  inclusions  are  comparatively  rare.  Glass  and 
slag  inclusions  are  observed  abundantly  in  glassy  extrusive  rocks. 
The  regularly  arranged  small  red  plates  which  cause  the  copper 
red  iridescence  of  sunstone  are  also  to  be  mentioned.  They 
are  generally  considered  to  be  hematite,  but  this  has  not  been 
definitely  proved. 

Orthoclase,  as  a  constituent  of  granite  and  quartz  porphyry, 
is  one  of  the  most  widespread  minerals  and  is  very  important  as 
a  source  of  potassium  for  the  soils.  It  is  also  very  common  in 
injected  schists,  but  is  rarer  in  contact  rocks.  It  is  only  a  subor- 
dinate constituent  of  clastic  rocks  in  which  it  is  rather  fresh. 

In  central  granite  the  orthoclase  is  entirely  fresh  and  is 
characterized  by  a  very  perfect  cleavage — adularia.  The  form 
known  as  sanidine  is  likewise  often  glassy,  but  it  has  a  more 
concoidal  fracture.  Ordinary  orthoclase  is  in  a  state  of  altera- 
tion, which  sometimes  begins  on  the  edge,  sometimes  in  the 
center  and  sometimes  in  certain  zones.  It  gives  rise  to  cloudy, 


(OO/) 


(010) 


FIG.  354.— Microcline,  Lattice  Structure. 


Fio.  355. — Microcline,  Explanation 
of  Lattice  Structure. 


scaly  aggregates  assembled  chiefly  along  the  cleavage  cracks. 
Ordinary  alteration  gives  rise  to  sericitic  substances,  the  double 
refraction  of  which  can  be  recognized  even  in  the  very  fine,  scaly 
particles.  Kaolinization  is  a  very  different  process  from  this 
and  is  entirely  local.  The  aggregates  resulting  from  it  have 
scarcely  any  double  refraction  at  all  and  are  generally  so  crumbly 
that  they  are  not  observed  in  a  thin  section  because  they  fall  out 
during  the  grinding. 


DESCRIPTIVE  SECTION 


329 


Microcline  is  very  similar  to  orthoclase  macroscopically  but 
they  can  be  differentiated  under  the  microscope  by  different 
behavior  in  polarized  light.  The  former  can  always  be  recog- 
nized beyond  a  doubt  by  the  characteristic  twinning  lamination 
known  as  lattice  structure,  Fig.  354.  This  distinctive  property 
appears  to  be  the  result  of  lamellar  twinning  according  to  the 
albite  law.  The  lamellae  do  not  lie  side  by  side  as  in  the  case  of 
the  plagioclases,  but  cut  each  other  in  the  manner  shown  in  Fig. 
355.  The  triclinic  character  of  microcline  appears  on  the  cleav- 
age plates  which  have  an  extinction  of  15°  and  show  an  imsym- 
metrical  interference  figure,  Fig.  356. 


doi)   a 


(101) 


(no) 


FIG.  356. — Microcline, 
Cleavage  Plate  Parallel  Base. 


(010) 


FIG.  357. — Anorthoclase. 


The  twinning  mentioned  for  orthoclase  is  also  observed  on 
microcline  and  likewise  the  intergrowth  with  albite — microcline 
perthite.  It  is  sometimes  found  in  very  large  crystals  occurring 
in  pegmatites  of  the  granite  series  or  of  the  soda  rocks.  It  often 
occurs  with  a  very  brilliant  red  or  green  color — amazonstone. 
It  is  widespread  as  a  rock  constituent  in  granites  and  injected 
schists  but  only  the  microscope  reveals  it  as  the  last  mineral 
to  separate  out.  It  only  shows  crystal  form  in  the  neighborhood 
of  cavities.  It  is  entirely  lacking  in  extrusive  rocks  but  the 
orthoclase  of  these  rocks  sometimes  shows  the  microcline  struc- 
ture as  a  result  of  pressure.  Microcline  is  also  very  common  in 
granular  soda  rocks  but  its  properties  are  much  less  regular. 

Its  great  resistance  to  weathering  is  noteworthy.  It  is  often 
found  unaltered  in  soil  and  in  kaolinized  granite  when  the  other 
feldspars  have  been  entirely  destroyed. 

Soda  orthoclase  or  anorthoclase  is  also  triclinic,  Fig.  357.     It 


330 


PETROGRAPHIC  METHODS 


is  confined  to  the  soda  rocks  in  which  it  replaces  orthoclase.  In 
some  of  these  rocks  it  shows  a  characteristic  rhombic  form, 
previously  mentioned,  while  in  others  it  shows  the  same  develop- 
ment as  orthoclase.  A  brilliant  bluish  iridescence  is  quite  com- 
mon. It  may  be  observed  in  thin  section  that  anorthoclase  is 
united  to  cryptoperthite  by  all  possible  transition  stages.  Twin- 
ning is  exceptionally  common.  However,  the  lamellar  structure 

according'to  the  different  laws 
is  so  fine  that  it  only  appears 
distinctly  in  thin  section. 
The  laminated  sections  do 
not  have  sharp  boundaries 
between  the  parts  and  these 
finally  grade  over  into  homo- 
geneous sections.  The  char- 
acteristic denticulated  laths 
in  the  ground  mass  of  certain 
soda  trachytes,  Fig.  358,  are 
especially  distinctive.  Be- 
sides the  above  phenomena, 

FIG.  358.— Soda  Feldspar  in  Bostonite  from  .         *     ,  ,          _ 

Conny  island,  Mass.,  u.  s.  A.  anorthoclase  is  distinguished 

from  orthoclase  by  a  smaller 

extinction  on  the  basal  pinacoid  and  a  larger  extinction  on  the 
side  pinacoid  running  up  to  10°.  It  also  has  a  smaller  optic  angle. 
All  alkali  feldspars  are  not  attacked  by  acids  except  hydro- 
fluoric, but  it  dissolves  them  quite  rapidly.  They  melt  with 
difficulty.  They  are  distinguished  from  the  plagioclases  by 
lower  indices  of  refraction,  even  when  the  twinning  lamination 
is  lacking  in  the  latter.  Investigation  of  the  powder  immersed 
in  benzonitrile  (n  =1.526)  is  very  useful  to  distinguish  the  feld- 
spars. Orthoclase  has  lower  indices  of  refraction  in  all  directions 
than  the  liquid;  anorthoclase  and  microcline  are  a  little  lower  in 
one  direction  and  only  a  trifle  higher  in  the  other;  all  the  plagio- 
clases have  higher  indices  in  all  directions.  The  alkali  feldspars 
are  easily  distinguished  in  thin  section  from  quartz,  cordierite, 
scapolite,  nephelite,  etc.,  by  their  indices  of  refraction  being 
lower  than  that  of  Canada  balsam. 

b.  Soda-lime  Feldspars  or  Plagioclase 

The  plagioclase  group  consists  of  crystallized  triclinic  mixtures 
of  soda  feldspar  or  albite,  A  b,  with  lime  feldspar  or  anorthite, 


DESCRIPTIVE  SECTION 


331 


An.     It  has  as  yet  not  been  positively  determined  whether  the 
plagioclases  represent  a  continuous  isomorphous  series  in  which 


/;,.-:-.7.::^^:^- 

•;i:,^:>-*.  \;  >;;:;: 


FIG.   359.— Albite. 


FIG.  360. — Oligoclase.  FIG.  361.— Andesine. 


FIG.  362. — Labradorite.  FIG.  363. — Bytownite.  FIG.  364. — Anorthite. 


the  two   silicates   may  grow  together  in   any   proportions,   or 
whether  only  certain  compounds  with  definite  composition  are 


332 


PETROGRAPHIC  METHODS 


formed.  The  latter  appears  the  more  probable.  At  any  rate, 
certain  types  given  in  the  table  are  much  more  common  than 
mixtures  that  might  be  placed  between  them.  Phenocrysts  in 
porphyric  rocks  are  well  bounded  crystallographically  as  are 
often  the  microlitic  individuals  of  the  ground  mass,  especially  if 
the  rock  contains  glass.  The  plagioclases  show  isometric  forms 
only  in  the  more  acid  rocks.  In  other  rocks  they  are  partly 


•20 


-jo 


-20 


-10 


FIG.  365. — Stereographic  Projection  of  the  Optical  Constants  of  the  Plagioclases. 


prismatic  along  the  a  axis  and  partly  tabular  parallel  to  the  side 
pinacoid,  particularly  in  the  more  basic  varieties.  Their  cross 
sections  are  predominantly  lath-shaped,  Fig.  366. 

The  basal  and  brachypinacoids,  the  two  principal  cleavage 
directions,  form  an  angle  of  about  93J°.  The  base  is  grooved  by 
regular  parallel  striations  produced  by  the  ever  present  twinning 
lamination  parallel  to  the  brachypinacoid — albite  law,  Fig.  367. 
These  striations  on  the  most  perfect  cleavage  face  are  the  only 
useful  macroscopic  distinction  from  orthoclase,  if  the  latter  does 
not  have  its  characteristic  red  or  brownish  color,  which  does  not 
occur  in  plagioclase.  The  twinning  lamination  is  also  the  best 


DESCRIPTIVE  SECTION 


333 


characteristic  under  the  microscope.  It  is  rarely  recognized  in 
thin  sections  by  the  reentrant  angles,  Fig.  368,  but  it  shows  up  in 
polarized  light  by  the  different  extinction  of  the  lamellae.  If  it  is 
lacking,  as  is  frequently  the  case  in  basic  plagioclase  of  eruptive 
rocks  and  very  often  in  contact 
rocks,  the  determination  of  the 
feldspar  is  very  difficult.  The 
lamination  is  quite  variable, 
sometimes  broad  and  some- 
times narrow.  The  former  ap- 
pears to  be  the  more  common 
in  plagioclase  rich  in  lime  and 
the  latter  in  those  consisting 
predominantly  of  soda.  A  fine 
system  of  lamellae  may  alter- 
nate with  one  band  which  oc- 
cupies most  of  the  crystal.  The 
section  is  regularly  laminated 
in  one  case,  and  in  the  other, 

there  are  a  few  fine  lamellae,  while  the  rest  of  the  crystal  ap- 
pears homogeneous.  Sometimes  the  stripes  pass  clear  through 
the  individual  and  sometimes  they  narrow  down  and  pinch  out 
entirely.  Where  orogenic  pressure  has  acted  on  the  plagioclase 
the  lamellae  .may  be  bent  or  broken  and  may  be  displaced. 

(00  f) 


Fia.  366. — Lath-shaped  Plagioclaae  in 
Trap,  Reykjavik,  Iceland. 


(010) 


Fia.  367.— Plagioclase, 

Albite  Twinning 

Lamellae. 


FIG.  368. — Twin  Lamination  after  the 

Albite  Law  in  a  Section  Parallel 

to  the  Macropinacoid. 


In  addition  to  this  most  frequent  twinning  there  is  often  another, 
likewise  lamellar,  according  to  the  pericline  law.  The  b  axis  is 
the  twinning  axis.  This  second  system  of  lamellae  crosses  the 
first  under  variable  angles,  but  approximates  a  right  angle  in  the 


334  PETROGRAPHIC  METHODS 

zone  of  the  b  axis,  Fig.  371,  page  338.  Such  polysynthetic  twin- 
ning may  be  combined  with  one  of  the  laws  for  orthoclase,  most 
frequently  the  Carlsbad,  and  the  appearance  becomes  quite  com- 
plicated. Globular  and  star-shaped  intergrowths  of  many  indi- 
viduals are  found  in  certain  rocks,  particularly  andesites  and 
tephrites.  By  their  regularity,  they  give  the  impression  of  a 
very  complicated  growth  of  twins. 

The  occurrence  of  isomorphous  layers  is  extraordinarily  fre- 
quent in  intermediate  eruptive  rocks  and  is  very  troublesome  in 
the  exact  determination  of  the  plagioclase.  The  different  zones, 
which  are  sometimes  very  narrow,  extinguish  differently.  The 
center  always  consists  of  a  more  basic  plagioclase  than  the  outer 
zones.  Twinning  lamination  continues  undisturbed  throughout 
all  the  zones.  Irregular  penetration  of  different  plagioclases 
also  occurs  in  eruptive  rocks.  Zonal  structure  is  very  wide- 
spread in  contact  rocks,  but  here  the  order  is  reversed  and  the 
center  is  more  acid  than  the  rest.  See  page  326  for  intergrowths 
with  orthoclase. 

The  plagioclases  are  always  colorless  in  thin  section.  They 
are  very  fresh,  particularly  in  unaltered  extrusive  rocks  where 
they  have  the  appearance  of  sanidine.  This  has  been  distin- 
guished as  microtine.  Abundant  inclusions  of  dark  glass,  often 
with  zonal  arrangement,  are  very  common  in  glassy  rocks.  Most 
of  the  acid  plagioclases  in  certain  Pentral  Alpine  granites  and 
tonalites  are  fresh  and  transparent,  but  they  are  often  filled  with 
a  large  number  of  microlites  so  that  they  appear  clouded  macro- 
scopically  and  become  transparent  only  in  very  thin  slides.  The 
microlites  are  well  bounded  crystallographically,  Fig.  274,  page 
264,  and  generally  they  are  orientated  at  random  in  the  fresh 
mass  of  the  feldspar.  Sometimes  they  are  muscovite,  sometimes 
sillimanite  or  garnet,  but  in  most  cases  they  are  members  of  the 
epidote  group  poor  in  iron.  These  plagioclases  are  light  reddish 
in  color  when  they  contain  garnet,  but  the  epidote  minerals  pro- 
duce a  light  greenish-yellow  color. 

Plagioclase  is  generally  clouded  in  the  eruptive  rocks  and  its 
alteration  is  quite  analogous  to  that  of  orthoclase  except  that 
plagioclase  containing  lime  is  more  easily  attacked,  and  epidote, 
calcite,  chlorite,  etc.,  frequently  occur  as  by-products  of  the 
alteration.  Pseudomorphs  of  hydrargillite  after  plagioclase  have 
been  found  locally  in  greatly  altered  rocks  and  replacement  by 
zeolite  is  not  at  all  uncommon  in  soda  rocks.  Brown  tabular 


DESCRIPTIVE  SECTION 


335 


microlites  with  parallel  orientation  occur  as  inclusions,  particu- 
larly in  basic  plagioclase.  These  are  often  the  cause  of  the  cha- 
toyancy.  Such  feldspars  sometimes  appear  brownish-black  mac- 
roscopically,  but  under  the  microscope  they  seem  to  be  filled  with 
dust.  This  appearance  may  also  depend  upon  similar  inclusions 
minutely  divided. 

The  two  end  members  of  the  plagioclase  series  are  the  rarest 
as  rock  constituents.  Albite  is  the  commonest  representative 
of  the  series  in  granite  pegmatites,  in  which  it  is  the  last  mineral 
to  crystallize.  It  occurs  as  a  rock  constituent  in  perthitic  inter- 
growth  with  orthoclase,  but  aside  from  this  it  is  a  rare  exception 
in  granites.  It  occurs  more  frequently  in  acid  members  of  the 
soda  series,  in  which  it  is  often  the  only  feldspar.  Clouded  indi- 
viduals of  it  are  present  in  the  corresponding  extrusive  rocks, 
especially  in  keratophyres.  Oligoclase  to  andesine  are  the  usual 
plagioclases  in  acid  and  intermediate  eruptive  rocks.  Labra- 
dorite  is  sometimes  quite  widespread,  for  example,  in  monzo- 
nite.  It  is  the  typical  feldspar  of  gabbro  and  trap,  sometimes 
passing  over  into  bytownite,  but  rarely  into  anorthite.  In 
general,  phenocrysts  in  por- 
phyrites  and  andesites  ap- 
proach labradorite,  while  more 
acid  plagioclase  predominates 
in  the  ground  mass. 

Plagioclase  is  much  fresher 
in  contact  rocks  and  is  usually 
clear  and  transparent.  Its 
lack  of  twinning  lamellae  has 
been  mentioned  above.  It 
shows  good  crystal  form  only 
in  granular  carbonate  rocks  in 
which  isolated  microscopic  in- 
dividuals of  albite  are  very 
widespread,  or  in  the  lime- 
silicate  fels  formed  near  them.  Here  anorthite  often  forms  large 
individuals.  The  Knoten  in  certain  Alpine  graphite  schists  are 
often  composed  of  rounded  crystals  of  albite.  These  are  more 
commonly  granular  aggregates  with  beautifully  developed  mosaic 
structure,  Fig.  369.  They  are  often  very  rich  in  inclusions. 

Under   the  influence   of  piezocontact   metamorphism,    those 
plagioclases  rich  in  lime  decompose  into  calcium-aluminium  sili- 


FIG.  369. — Albite  in  Green  Schist.     Tummel- 
bachtal,  Grossvenediger.    Mosaic  Structure. 


336  PETROGRAPHIC  METHODS 

cates  like  clinozoisite,  garnet,  etc.,  and  into  a  granular  mosaic 
of  clear  plagioclase  low  in  lime.  The  higher  the  pressure  during 
the  recrystallization  the  more  the  latter  approaches  albite.  It 
is,  therefore,  almost  always  pure  albite  in  the  schistose  masses 
of  the  Alps.  Under  lower  pressure  the  plagioclase  simply  recrys- 
tallizes  so  that  in  the  normal  contact  metamorphism  of  diabase 
the  original  lath-shaped  crystals  retain  their  outward  form,  but 
consist  of  a  granular  aggregate  of  the  same  feldspar. 

Another  type  of  alteration,  effecting  especially  basic  plagio- 
clase in  eruptive  rocks,  is  quite  analogous  in  its  entire  character 
to  piezocontact  metamorphism.  This  is  saussuritization  and  is 
usually  accompanied  by  uralitization  of  the  pyroxene.  Macro- 
scopically  the  saussurite  appears  to  have  the  form  of  the  feldspar, 
but  it  has  no  trace  of  cleavage  but  consists  of  dense,  hard,  heavy 
aggregates  with  splintery  to  conchoidal  fracture  and  is  character- 
ized by  great  tenacity.  Its  color  is  greenish  or  yellowish,  passing 
over  into  almost  pure  white.  It  has  a  clouded  appearance  under 
the  microscope  even  in  very  thin  slides  because  of  the  small  size 
of  the  irregular  individuals  with  high  indices  of  refraction.  These 
minerals  are  most  frequently  members  of  the  epidote  and  garnet 
groups  poor  in  iron,  vesuvianite,  lawsonite,  prehnite  often 
penetrated  by  hornblende  needles  or  flakes  of  chlorite.  The 
host  of  all  this  aggregate  seems  to  be  plagioclase  similar  to  albite. 
A  definite  conclusion  concerning  the  composition  of  saussurite 
is  very  difficult  because  of  the  poor  development  of  the  various 
constituents  and  the  similarity  of  their  optical  properties.  Pseu- 
domorphs  of  chlorite  or  talc  after  different  feldspars  are  found 
locally  as  rare  formations. 

Acid  plagioclase  is  effected  by  acids  just  as  the  alkali  feldspars 
are.  Basic  plagioclase  is  readily  attacked  by  hot  hydrochloric 
acid  and  the  richer  in  lime  it  is  the  more  easily  it  is  gelatinized. 
Acid  members  are  rather  difficultly  fusible  before  the  blowpipe 
and  the  basic  are  almost  infusible. 

In  many  cases  it  Jhklesirable  to  determine  the  plagioclase  accurately 
because  of  the  importance  of  this  mineral  in  the  classification  of  the  eruptive 
rocks.  The  most  reliable  means  for  this  purpose  is  quantitative  chemical 
analysis,  but  it  requires  a  great  deal  of  time  and  frequently  enough  pure 
material  cannot  be  obtained  for  it.  Determination  of  the  silica  content 
alone  would  be  quite  useful  as  will  be  seen  from  column  6,  Table  16.  The 
difficulty  here  is  the  same  as  in  the  preceding  case. 

A  number  of  qualitative  chemical  tests  can  be  made  on  small  fragments 
that  can  be  easily  isolated  in  a  pure  condition  from  the  rock,  but  they  do 


DESCRIPTIVE  SECTION 


337 


not  give  very  definite  results.  Thus  attempts  have  been  made  to  estimate 
the  soda  content  of  a  feldspar  by  the  color  produced  by  the  mineral  in  a 
flame  or  by  the  relative  amounts  of  sodium  fluosilicate  and  calcium  fluo- 
silicate  in  the  residue  after  the  mineral  has  been  decomposed  by  hydrofluoric 
acid.  The  end  members  may  be  distinguished  by  their  behavior  toward 
hydrochloric  acid.  Determination  of  the  specific  gravity,  column  7,  gives 
better  results  than  the  above  methods.  A  very  pure  minute  grain  can  be 
used  for  this  purpose. 

A  few  of  the  numerous  optical  methods  for  the  determination  of  the  plagio- 
clase  are  much  more  reliable  than  all  the  above  tests.  The  optical  orienta- 
tion of  the  plagioclases  is  known  in  a  general  way,  although  there  is  some 
disagreement  about  minor  details  so  that  determinations  by  different 
methods  have  given  different  results.  This  does  not  appear  to  be  due  to 
the  method  used  alone  but  is  dependent,  to  a  certain  extent  at  least,  upon 
varying  optical  properties  caused  by  uncontrollable  contingencies.  Optical 
methods  cannot  replace  quantitative  chemical  analysis  if  there  is  suitable 
material  for  it.  They  often  give  better  results  than  the  latter,  however, 
under  the  conditions  in  which  a  petrographer  finds  them. 

The  optical  properties  of  the  plagioclases  have  been  thoroughly  in- 
vestigated because  of  the  importance  of  these  minerals  in  the  classification 
of  the  eruptive  rocks.  Figs.  359  to  364,  page  331, 
and  the  projection  hi  Fig.  365  give  a  general  survey 
of  these  properties.  A  and  B  in  these  figures  are 
the  optic  axes  and  a,  /?,  7*  are  the  principal  vibra- 
tion directions.  The  index  1,  Fig.  365,  refers  to 
albite,  2  to  oligoclase,  etc.,  corresponding  to  the 
six  figures  359  to  364.  In  general,  mean  values 
are  used  as  the  basis  for  construction  so  that  there 
are  small  deviations  from  the  true  relationships,  but 
a  synopsis  of  the  whole  can  be  obtained  from  the 
figure. 

It  is  understood  that  in  all  the  determinations 
with  these  figures  or  with  table  16  belonging  to 
them,  the  orientation  of  the  section,  if  it  is  not  a 
cleavage  piece,  is  determined  much  more  accurately 
in  convergent  polarized  light  than  in  parallel  light, 
but  for  other  purposes  only  measurements  in  paral- 
lel polarized  light  yield  good  results.  The  optic 
angle  of  plagioclase  is  so  large  that  its  measure- 
ment and  the  determination  of  the  optical  charac- 
ter even  with  an  immersion  system  give  doubtful 
results.  The  dispersion  is  so  weak  that  it  cannot 

be  positively  determined  in  thin  section.  The  double  refraction  is  almost  the 
same  for  all  members  except  the  most  basic.  These  properties  are  therefore 
of  no  use  to  distinguish  the  different  members  of  the  plagioclase  group. 

The  most  important  optical  methods  for  determining  them  are  the  fol- 
lowing : 

1.  The  oldest  and  simplest  is  the  method  of  determining  the  extinction 
angle  on  cleavage  plates  parallel  to  P=  {001}  and  M=  {010} ;  see  table  16, 
22 


FIG.  370. — Sign  of  the  Ex- 
tinction in  Plagioclase. 


338 


PETROGRAPHIC  METHODS 


(004) 


fOJO) 


Fia.  371. — Orientation  of  a  Plagioclase 
Crystal  Elongated  along  the  a  Axis. 


columns  16  and  17.  The  significance  of  the  sign  in  this  determination  is 
explained  by  Fig.  370.  This  method  gives  results  rapidly  if  the  individuals 
of  plagioclase  are  large  and  fresh.  It  can  also  be  used  on  very  small  in- 
dividuals under  certain  circumstances.  A  small  fragment  of  rock  is  crushed 
between  two  object  glasses  and  the  resultant  fine  sand  is  used  for  the  in- 
vestigation. The  best  cleavage  is  parallel  to  the  basal  pinacoid,  but  the 
cleavage  plates  are  more  often  parallel  to  the  brachypinacoid  on  account 
of  the  lamellar  development  of  the  crystal  parallel  to  this  face.  The 
interference  figures  of  such  plates  are  not  characteristic  except  for  the  end 
members. 

2.  All  sections  perpendicular  to  the  brachypinacoid,  i.e.,  the  twinning 
plane  of  the  albite  law,  show  extinction  of  the  individuals  symmetrical  to 
the  trace  of  the  twinning  plane.  Such  sections  may  be  recognized  in  a  slide 

by  the  symmetrical  extinction  of  the  two 
parts.  The  maximum  extinction  in  this 
zone,  column  21,  can  be  found  approxi- 
mately by  measuring  a  large  number  of 
sections  showing  symmetrical  extinction 
in  a  slide.  These  measurements  give 
valuable  results,  particularly  with  minute 
individuals  in  the  ground  mass  of  ex- 
trusive rocks,  provided  that  all  the 
plagioclase  cross  sections  do  not  have 
about  the  same  orientation  on  account  of 
flowage  of  the  magma.  «tf  '  "'M 

Sections  perpendicular  to  the  a  axis,  column  20,  also  have  a  special  signifi- 
cance. These  sections  are  rhombic,  nearly  quadratic,  in  outline  on  the 
small  individuals  due  to  equal  development  of  the  basal  and  brachypina- 
coids,  Fig.  371.  Cleavage  parallel  to  the  base  and  the  albite  twinning 
lamellae  are  both  perpendicular  to  these  sections.  The  extinction  on  such 
sections  is  a  splendid  means  for  determining  plagioclase  microlites  that  can 
scarcely  be  determined  by  other  methods.  If  the  positive  or  negative 
direction  of  the  extinction  cannot  be  determined  the  results  may  have  a 
two-fold  meaning,  at  least  with  the  acid  plagioclases. 

3.  Fouque  Method. — Determination  of  the  extinction  angle  in 
sections  in  which  the  optical  orientation  can  be  easily  and  quite 
accurately  determined  by  investigation  in  convergent  polarized 
light,  is  very  important.  The  best  sections  are  those  perpendic- 
ular to  one  of  the  two  bisectrices,  columns  18  and  19.  Sections, 
orientated  favorably  enough,  can  be  found  quite  readily  with  a 
little  practice,  in  almost  any  slide.  The  index  of  refraction  of  the 
plagioclase  is  compared  with  that  of  the  Canada  balsam  and  then 
by  studying  the  figures  359  to  364,  an  idea  of  the  direction  of  the 
cleavage  or  twinning  lamellae  in  the  section  in  question  is  obtained. 
Sections  are  then  found  corresponding  to  this  conception,  these 
showing  an  interference  color  about  half  as  high  as  the  highest 


DESCRIPTIVE  SECTION  339 

color  possible  in  plagioclase.  When  a  section  is  found  as  nearly 
perpendicular  to  a  bisectrix  as  possible,  its  character  is  deter- 
mined with  a  gypsum  test  plate,  regardless  of  whether  it  is  the 
acute  or  obtuse  bisectrix.  On  sections  perpendicular  to  the 
negative  bisectrix,  the  angle  between  the  optic  plane  and  the 
trace  of  the  twinning  plane  can  always  be  measured.  This  is 
apparent  in  Figs.  359  to  364.  Sections  perpendicular  to  the 
positive  bisectrix  give  the  angle  between  the  cleavage  parallel 
to  the  base  and  the  optic  plane,  while  the  twinning  lamellae  are 
so  oblique  that  they  cannot  be  observed  at  all  or  are  not  very 
sharp.  Column  18  gives  the  angles  between  the  optic  plane  and 
the  twinning  lamellae  of  the  albite  law  and  the  values  in  column  19 
are  the  angles  between  the  optic  plane  and  the  cleavage  parallel 
to  the  base.  Both  of  these  values  together  furnish  a  distinctive 
characteristic  for  a  feldspar,  while  only  one  of  them  is  not  suffi- 
cient for  the  determination,  in  many  cases. 

4.  In  double  twins  where  the  albite  and  Carlsbad  laws  occur  simultan- 
eously, there  are  four  conjugate  extinction  angles  for  the  different  parts  in 
each  section  where,  these  two  laws  appear  distinctly.     If  1  and  2  are  the  two 
individuals  twinned  according  to  the  Carlsbad  law,  each  contains  albite 
lamellae  I'  and  2'.     Each  member  of  the  pldgioclase  series  has  a  character- 
istic value  for  2  and  2'  corresponding  to  a  certain  value  for  1  and  1'.     This 
can  be  deduced  from  the  stereographic  projection.     To  take  this  up  in 
detail  would  lead  too  far  for  this  text. 

5.  The  mean  value  of  a  number  of  extinction  angles,  measured  from  the 
twinning  plane  on  several  random  sections  in  a  slide,  also  furnishes  some- 
what of  an  indication  of  the  kind  of  plagioclase.     Thus  more  than  half  of 
all  sections  of  anorthite  show  an  extinction  between  31°  and  50°  and  oligo- 
clase  between  0°  and  5°.     It  follows  from  this  that  the  twinning  lamellae 
are  not  distinctly  seen  in  the  latter  case  where  the  difference  in  extinction 
of  the  two  parts  is  so  small,  while  in  the  former  case  the  twinning  appears 
very  distinctly. 

6.  As  shown  in  Fig.  365,  one  of  the  optic  axes  in  basic  plagioclase  is  not 
very  much  inclined  to  the  vertical  axis.    The  point  at  which  this  axis  emerges 
in  each  component  of  a  double  twin  according  to  albite  and  Carlsbad  laws 
can  be  determined  from  the  interference  figure  if  the  section  is  not  greatly 
inclined  to  the  vertical  axis.     <p  and  X  can  be  determined  from  this  with 
respect  to  the  twinning  plane  and  these  values  can  be  used  to  determine 
the  plagioclase  from  Fig.  365  in  which  the  values  are  indicated.     Such 
sections  of  the  members  richer  in  sodium  do  not  show  the  emergence  of  an 
optic  axis.     This  method  therefore  has  very  little  practical  significance. 

7.  Schroeder  van  der  Kolk  Method. — One  method,  which  often 
gives  rise  to  good  results,  is  based  upon  the  difference  in  the 
indices  of  refraction  for  the  different  plagioclases.  Whether  the 


340  PETROGRAPHIC  METHODS 

plagioclase  is  acid,  intermediate  or  basic  can  be  very  rapidly 
determined  by  comparing  its  index  with  that  of  Canada  balsam. 
More  accurate  results  can  be  obtained,  especially  with  inter- 
mediate members,  if  the  different  elasticity  axes  of  the  plagio- 
clase are  compared  with  the  two  vibration  directions  of  the 
quartz  grains  lying  beside  it,  column  8,  Table  16.  This  method  is 
especially  valuable  for  the  investigation  of  rock  powders,  sands, 
and  soils  in  which  the  minutest  flecks  of  the  feldspar  are  sufficient 
to  obtain  perfect  results.  A  series  of  liquids  can  be  arranged  in 
well  stoppered  phials.  The  liquids  must  not  decompose  too 
easily  and  they  must  not  be  of  such  a  nature  as  to  be  impaired 
very  much  by  use.  They  are  so  chosen  that  the  index  of  refrac- 
tion of  the  liquid  lies  between  those  of  two  principal  members  of 
the  plagioclase  series.  A  grain  of  the  feldspar  to  be  investigated 
is  placed  upon  an  object  glass  in  a  drop  of  the  liquid,  the  index  of 
refraction  of  which  has  been  previously  determined.  The  cover- 
glass  is  laid  on  and  the  illuminating  cone  of  light  is  narrowed 
down  as  much  as  possible.  Then  the  index  of  refraction  of  the 
two  vibration  directions  of  the  plagioclase  is  compared  with  that 
of  the  liquid. 

The  liquids  used  are  organic  because  solutions  of  inorganic 
salts,  described  in  Part  I,  page  37,  change  their  indices  rapidly 
upon  evaporation.  The  purity  of  the  liquids  and  therefore  the 
indices  of  refraction  are  rather  variable  so  that  the  indicators 
must  be  tested  with  a  Bertrand  refractometer  and  the  true  index 
determined  before  the  series  is  made  up.  The  values  given  below 
were  determined  by  Weinschenk  for  liquids  obtained  from  Ben- 
der and  Hobein  in  Munich.  The  variations  for  different  tests 
from  other  sources  amounted  to  about  ±0.002  to  0.004. 

The  following  serve  to  distinguish : 


Alkali  feldspar  from  plagioclase:  Benzonitrile n  =  1.526 

Albite  from  oligoclase :  Eugenol n  =  1 . 540 

Bromtoluol n  =  l.  550 

Nitrobenzol n  =  1 .  552 

Andesine  from  labradorite:  Anethol n  =  l  .558 

Labradorite  from  bytownite:  Monobrom  benzol.  .  .  .n  =  1.560 
Bytownite  from  anorthite:  Orthotoluidine n  =  1 . 575 


£&M?*W     XX  V/JLJ-I.      \SAX£^\S VA«-*«V-»  •       JLJ  l&g 

Oligoclase  from  andesine :  < 


It  is  necessary  to  carry  out  these  investigations  in  sodium 
light  on  account  of  the  strong  dispersion  of  the  liquids  and  the 
great  difference  in  dispersion  between  them  and  the  feldspars. 
The  above  indices  are  given  for  sodium  light. 


DESCRIPTIVE  SECTION  341 

8.  The  Fedorow  universal  stage  was  originally  constructed  for  determining 
the  feldspars;  see  Part  I,  page  127.     The  universal  stage  is  placed  on  the  stage 
of  the  microscope,  with  its  principal  axis  of  rotation  exactly  parallel  to  the 
vibration  direction  of  one  of  the  nicols.     The  horizontal  axis  of  the  inner 
stage  will  then  lie  parallel  to  the  other  vibration  direction.     The  thin  sec- 
tion is  placed  on  a  small  round  object  glass  and  fastened  on  the  glass  stage 
with  a  drop  of  glycerine.     The  feldspar  to  be  investigated  is  centered  and  a 
plano-convex  lens  is  placed  over  it  and  one  under  the  stage,  contact  being 
made  with  glycerine.     Then  the  glass  stage  is  rotated  until  the  trace  of  the 
twinning  plane  is  parallel  to  the  horizontal  axis  of  the  inner  stage.     Then 
it  is  rotated  on  this  axis  until  the  two  individuals  of  the  albite  law  have  the 
same  interference  color,  i.e.,  they  extinguish  symmetrically  with  respect  to 
the  twinning  plane.    The  angle  of  rotation  is  read  and  plotted  in  the  stereo- 
graphic  projection.     Then  the  inner  stage  is  rotated  on  both  of  its  axes 
until  one  part  of  the  twin  remains  dark  when  the  nicols  are  rotated.     The 
pole  of  the  axis  which  emerges  perpendicularly  is  read.     The  pole  of  the 
other  optic  axis  is  determined  in  a  similar  manner.     The  position  of  A  and  B 
and  of  a,  /?  and  f  relative  to  the  twinning  plane  in  the  albite  law  is  thus 
determined  and  the  plagioclase  can  be  determined  directly  from  that. 

9.  A  Wallerant  total  reflectometer,  Part  I,  page  40,  can  also  be  used  to 
determine  the  plagioclase.     The  polarizer  is  pushed  out  and  the  critical 
angle  of  total  reflection  is  measured. 

Biaxial  Zeolites  (17) 

All  zeolites  are  colorless  of  themselves,  but  they  are  sometimes  colored 
yellowish-brown  to  red  by  inclusions  of  ferric  hydrates  or  brownish  by 
organic  substances.  They  all  have  low  indices  of  refraction,  lower  than 
Canada  balsam.  The  double  refraction  corresponds  somewhat  to  that  of 
quartz.  It  is  much  lower  in  phillipsite  and  harmotome  and  unusually 
high  in  thomsonite.  The  latter  does  not  appear  similar  to  the  other 
zeolites  in  thin  section  and  can  be  determined  as  such  only  by  its  solubility 
in  hydrochloric  acid  and  by  its  becoming  cloudy  when  slightly  heated. 
Laumontite  is  another  of  the  zeolites  given  in  the  table,  which  is  somewhat 
peculiar.  It  gives  up  a  part  of  its  water  at  ordinary  temperature  and  is, 
therefore,  always  cloudy.  Some  other  zeolites  show  the  same  phenomenon. 

The  zeolites  rarely  form  granular  aggregates,  but  phillipsite  and  harmo- 
tome may.  Most  of  them  appear  tabular  like  heulandite  or  radial  like 
epistilbite  and  desmine.  When  they  are  macroscopically  visible  they  nearly 
always  show  the  latter  form  of  aggregates  and  this  together  with  the  perfect 
cleavage  is  sufficient  for  the  determination. 

Since  the  indices  and  double  refraction  of  the  various  zeolites  are  about 
constant,  the  optical  character  of  the  principal  zone  can  be  used  to  distinguish 
some  of  them,  particularly  natrolite  from  scolesite.  The  latter  is  also 
characterized  by  a  small  optic  angle  and  strong  dispersion  of  the  optic  axes. 
Mesolite  stands  between  these  two  chemically  and  also  takes  an  intermediate 
position  with  respect  to  the  optical  properties. 

Oblique  extinction  serves  to  distinguish  monoclinic  zeolites  from  the 
orthorhombic.  It  is  small  in  the  minerals  of  the  stilbite  group  and  natrolite, 


342  PETROGRAPHIC  METHODS 

medium  in  scolesite,  laumontite  and  phillipsite  and  quite  large  in  harmo- 
tome.  Differentiation  of  the  zeolites  is,  however,  very  difficult  and  their 
significance  as  rock  constituents  has  by  no  means  been  determined. 

All  zeolites  are  typical  products  of  thermal  activity.  They  occur  chiefly 
in  amygdaloids,  especially  near  the  contacts  of  eruptive  rocks  and  frequently 
form  beautiful  crystals.  They  are  not  rare  as  infiltrations  in  tuffs  and  are 
also  found  in  sediments  in  the  neighborhood  of  eruptive  rocks.  Whatever 
their  occurrence,  the  principal  associate  is  chalcedony  and  the  other  modifi- 
cations of  silica.  The  difficulty  qf  distinguishing  the  zeolites  from  each  other 
is  increased  by  the  presence  of  these  minerals,  which  are  very  similar  opti- 
cally. The  determination  is  much  easier  when  they  occur  as  secondary  de- 
posits in  cavities  than  when  they  occur  in  decomposed  soda  rocks.  In  the 
latter  case  all  kinds  of  zeolites  occur  in  aggregates  similar  to  ice  flowers  or  in 
dense  pseudomorphs  after  nepheline,  leucite,  sodalite  minerals  and  feldspar. 
These  are  classed  together  as  spreustein.  Determination  of  the  different 
zeolites  is  impossible  in  such  an  occurrence.  They  can  be  shown  to  be  zeo- 
lites by  their  solubility  in  hydrochloric  acid  and  by  the  cloudiness  of  the 
section  when  it  is  heated  very  gently. 


DESCRIPTIVE  SECTION 


343 


§  is 
• 


S        "cij 
SB       a 


344 


PETROGRAPHIC  METHODS 


ISOTROPIC 


Page 

Group 

Minerals 

Ciuemical 
composition 

Crys- 
tal 
system 

Cleav- 
age 

Development 

Sp.  gr.    B 

213 

Perovskite 

CaTiO3 

C 

(100) 

Octahedral 
Hexahedral 
Granular 

4.0      5 

213 

Sphalerite 

ZnS 

C 

110 

Granular 

3.9 
to       3 
4.2 

214 

Garnet  Group 

Almandine 

Fe3Al2Si3012 

C 

(110) 

Dodecahedral 

Less  often 
tetragonal 
trisoctahedral 

Granular 

4.0 
to 
4.3 

Pyrope 

(MgFe)3Al2 
Si3012 

3.75 

Grossularite 

Ca3Al2Si3O12 

3.45 
to 
4.1 

Topazolite 

Ca3Fe2Si3012 

Melanite 

Ca3Fe2 
(SiTi)3012 

4.3 

218 

a 

Chrome  spinel 

(Fe  Mg) 
(Al  Cr)204 

C 

Octahedral 
Granular 

4.1 
to       1 
4.5 

Iron  spinel 

(Fe  Mg) 
(AlFe)204 

3.9 

Spinel 

MgAl2O4 

3.6 

219 

Periclase 

MgO  Contains  iron 

C 

100 

Octahedral 
Granular 

3.65 

219 

Boracite 

Mg7Cl2B16030 

C 

Hexahedral 
Tetrahedral 

2.9        7 

219 

Leucite 

KAlSi2O6 

C 

Tetragonal 
trisoctahedral 
Rounded 

2.5      c 

220 

|  /  acid 
£3  ^  basic 

Variable 

A 

2.25 
to 

2.7 

222 

Analcite 

NaAlSi,O6  +  aq. 

C 

100 

Granular 
Tetragonal 
trisoctahedral 

2.20     .1 

DESCRIPTIVE  SECTION 


MINERALS. 


345 
TABLE  1 


(Color 

n 

Solubility 

Remarks 

Gray 
Violet 
Brown 

2.38 

Only  soluble 
in  cone,  sul- 
phuric acid 

Optical  anoma- 
lies 
Lamination 

Yellow 
Brown 

2.37 

Soluble  in 
cone,  nitric 
acid 

Zinc 
coating  on 
charcoal 

Almost 
colorless  to 
reddish 

1.76 

to 
1.81 

Difficultly 
soluble  in  hy- 
drochloric acid 
Easily 
attacked 
after  melting 

Often  rich  in 
inclusions 
Siebstructure 

Light      red 

1.750 

Kelyphite 
border 

Variable 
Frequently 
colorless 
Reddish 
Brownish 

1.750 
to 
1.784 

Optical  anoma- 
lies 
Division  into  • 
segments 

Brown 

1.856 

Zonal  structure 

Brown 

approx. 
2.0 

Not  attacked 
by  acids  and 
molten  alkali 
carbonate 
Decomposed 
by  potassium 
bisulphate 

Green 

1.750 

Colorless 

1.720 

Colorless 
.  Light  green 

1.736 

Difficultly 
soluble 

Often  altered  to 
serpentine   and 
brucite 

Colorless 

1.667 

Difficultly 
soluble 

Optical  anomalies 
Lattice  structure 

Colorless 

1.509 

Gives  powdered 
silica  with  hy- 
drochloric acid 

Optical  anomalies 
Lattice  structure 
Inclusions 

Colorless 
Brownish 

1.49 
to 
1.63 

Variable 

Often  fluidal 

Colorless 

1.488 

Forms  gelatin- 
ous silica  in  hy- 
drochloric acid 

Optical 
anomalies 
Segments 

Explanation  of  the  Tables. 

The  first  vertical  column  gives  the 
page  in  the  book  on  which  the  mineral 
is  described.  Under  the  heading 
"Chemical  Composition"  frequently 
only  the  principal  constituents  are 
given  and  the  formula  is  simplified  so 
that  the  chemical  constitution  of  the 
mineral  is  expressed  only  in  a 
general  way.  In  the  "Crystal  Sys- 
tem" column,  A  =  amorphous,  C  = 
cubic,  H  =  hexagonal,  T  =  tetragonal, 
O  =  orthorhombic,  M  ==  monoclinic,  Tr 
=  triclinic.  The  symbols  for  the  cleav- 
age forms  are  written  differently  from 
the  customary  usage:  a  symbol  in 
parenthesis  indicates  imperfect  cleav- 
.age,  without  them  only  distinct  cleav- 
age, and  underscored  it  is  perfect 
cleavage.  Sp.  gr.  =  specific  gravity,  and 
H  at  the  top  of  a  column  is  for  hard- 
ness. 

The  values  given  for  specific  gravity, 
index  of  refraction  (n  or  a,  /?,  r)  and 
double  refraction  are  only  average 
values  and  may  vary  greatly  especi- 
ally in  mixed  isomorphous  groups. 
When  such  values  have  been  accurate- 
ly measured,  as  in  the  case  of  scapolite 
and  epidote,  the  maximum  and  mini- 
mum are  given. 

Color  also  may  vary  within  wide 
limits  in  one  and  the  same  mineral 
Only  the  colors  that  are  most  import- 
ant for  rocks  are  given  in  that  column 
and  then  only  those  that  can  be  ob- 
served in  thin  section.  Pleochroism  is 
even  more  variable  than  the  regular 
color  of  the  mineral.  When  it  is 
characteristic  and  more  or  less  con- 
stant it  is  indicated  under  the  indices 
of  refraction.  Otherwise  the  absorp- 
tion formula,  which  is  much  more 
constant,  is  given.  The  character  of 
the  double  refraction  of  the  mineral  is 
indicated  under  Chm  and  that  of  the 
principal  zone  under  Chz. 

In  the  column  "Optical  Orienta- 
tion" the  6  axis  is  always  given  in  bi- 
axial crystals.  It  can  then  be  seen  at  a 
glance  whether  the  plane  of  the  optic 
axes  is  parallel  or  perpendicular  to  the 
plane  of  symmetry  in  monoclinic  min- 
erals. The  extinction  angle  is  general- 
ly measured  from  the  vertical  axis;  / 
after  the  number  indicates  that  it  is 


346                      PETROGRAPHIC  METHODS 
ISOTROPIC 

Page 

Group 

Minerals 

Chemical 
composition 

Crys- 
tal 
system 

Cleav- 
age 

Development 

Sp.gr. 

223 

Sodalite  Group 

Sodalite 

+  NaCl 

O 

+  Na2S04 

03 

£ 

eo 

+  (Na2Ca)SO4 

C 

(110) 

Dodecahedral 
Granular 

2.3 
to 
2.5 

- 

Noselite 

Haiiyne 

224 

Opal 

SiO2  +  z  aq. 

A 

2.2 

225 

Fluorite 

CaF2 

C 

111 

Granular 

3.2 

DESCRIPTIVE  SECTION  347 

MINERALS.— Continued.  TABLE  2 


Color 

n 

Solubility 

Remarks 

1.483 

Regularly  arrang- 
ed rod-like  in- 

I 

Colorless 

Form  gelatin- 
ous silica 

clusions 
Black  corrosion 

1.495 

Blue 
(Yellow,  Green) 

with  hydro- 
chloric acid 

borders 
Often  altered  to 

1.504 

zeolite 

Difficultly    fusible 

Colorless 

1.46 

Soluble  in 
potassium 
hydroxide 

Index  of  refrac- 
tion variable 
Often  anomalous 
double  refraction 

With  sulphur- 

Colorless 

1  434 

ic  acid  it  gives 

Phosphorescen  t 

Violet 

off  hydrofluoric 

when  heated 

acid  vapor 

Explanation  of  the  Tables — Continued, 
toward  the  front  or  in  the  obtuse  angle 
/?;  r  means  to  the  rear.  The  expres- 
sions "difficultly  soluble"  and  "insol- 
uble" refer  to  hydrochloric  acid  as  a 
solvent  unless  otherwise  indicated.  The 
next  column  indicates  the  most  impor- 
tant phenomena  that  are  sometimes 
observed.  Thus  "optical  anomalies" 
only  means  that  the  mineral  is  some- 
times anomalous  and  not  that  it  al- 
ways occurs  that  way.  The  meaning  of 
the  various  columns  in  table  15,  which 
includes  the  feldspars,  may  be  detT- 
mined  from  the  text.  Remarks  in 
parentheses  refer  to  rarer  and  different- 
ly developed  varieties  except  in  the 
cleavage  column  as  explained  above 
and  in  the  absorption  column  where 
the  parentheses  indicate  that  the  ab- 
sorption is  not  a  characteristic  ear  mark 
of  the  mineral,  p^ti  or  u^o  in  the 
dispersion  column  indicates  that  the 
dispersion  of  the  optic  axes  is  extra- 
ordinarily strong. 


348  PETROGRAPHIC  METHODS 

ROCK-FORMING  MINERALS,  OTHER  THAN  THE  CUBIC, 
INDEX  OF  REFRACTION,  DOUBLE 


Double  refraction  -> 
Index  of  refraction  | 

<  0.005 

to  0.010 

to  0.015 

to  0.020 

+ 

,;-•     I           ETC 

Tridymite 
Zeolites  -> 
Apophyllite  | 

Quartz 
Gypsum 

<-  Zeolites 

Hydromagnesite 

Nepheline 
Apophyllite  * 

Chalcedony 
Cordierite 
Zeolites  -> 

<-  Zeolites 

+ 
1.55 

Pennine 

Quartz 
Chrysotile 
Clinochlore  -» 

Wagnerite 
<-  Clinochlore 

Alunite 
Hydrargillite 

to 
1.60 

Margarite 
Beryl 
Kaolin 
Antigorite  ->• 

Scapolite  •*• 
<-  Antigorite 

+ 
1.60 

Melilite       * 
Eudialyte 

Celestite 
Topaz     -* 

Barite 
*-  Topaz 

Prehnite  •*• 
Pargasite 

1.65 

Apatite 
Melilite  t 
Eucolite 

Andalusite 
Wollastonite 

Tourmaline  \ 
Monticellite   ^ 

+ 
1.65 

Zoisite 
Thulite 

Enstatite 

Mosandrite 

Lawsonite 
Rinkite 
Spodumene 

to 
1.70 

Riebeckite 
Arfvedsonite 
Orthite  \ 

Dumortierite 
Gehlenite 
Axinite 
Margarite 

Xanthophyllite 
Prismatine 

Monticellite  t 
Glaucophane 

+ 
1.70 

to 

Clinozoisite  -> 
Chloritoid  -> 

Chrysoberyl 
Staurolite 
Serendibite 

<-  Clinozoisite 
<-  Chloritoid 

1.80 

Vesuvianite 
Sapphirine 

Corundum 

Epidote'    ->• 
Cyanite     -> 
Hypersthene 

<-  Cyanite 

+ 
>1.80 

Wurtzite 

- 

DESCRIPTIVE  SECTION 


349 


EXCEPT  THE   FELDSPARS,   ARRANGED    ACCORDING  TO 
REFRACTION,  AND  OPTICAL  CHARACTER.  TABLE  3 


to  0.025 

to  0.030 

to  0.050 

to  0.075 

to  0.100 

>0.100 

Wavellite 

Thomson!  te 

Cancrinite 

» 

Brucite 

Anhydrite 

,  Nontronite 
;  Bertrandite 

* 

<-  Scapolite 
Pyrophyllite 
Mica    | 

Talc 

Anthophyllite 
i  Crocidolite 

Rosenbuschite 

<-  Prehnite 
Pectolite 
Humite 

Gedrite 
Hornblende 
Carpholite 

Tremolite 
Actinolite 
Lazulite 

Mica   * 

Calcite 
Dolomite 
Magnesite 

!  Diallage 
Fassaite 
Sillinaanite 

Forsterite 
Diopside 
Jadeite 

Oli  vine 

.— 

]  Augite 

\  Tourmaline 
Datolite 

Aragonite 

Aegirine  augite 

Diaspore 
Monazite 

Astrophyllite 

Xenotime 

Lavenite 
\  Orthite 

Piemontite 
Aegirine 

•«-  Epidote 
Basaltic 
hornblende 

Zircon 

Cassiterite 
Pseudobrookite 
Titanite  -> 

Rutile 
Brookite 
+  Titanite 

Fayalite 

Anatase 

Goethite 
Baddeleyite 

Sulphur 

350 


PETROGRAPHIC  METHODS 


UNIAXIAL 


Page 

p. 
O 

Minerals 

Chemical 
composition 

Crystal 
system 

Cleav- 
age 

Develop- 
ment 

Sp.  gr. 

H 

Color 

Brown 

227 

Rutile 

TiO2 

T 

110 
(100) 

Prismatic 
•Granular 

4.25 

6.5 

Yellowish 
Grayish- 

violet 

111 

Granular 

Colorless 

229 

Anatase 

Ti02 

T 

Tabular 

3.85 

6 

Blue 

001 

Pyramidal 

Yellowisl 

Granular 

Yellowish 

230 

Cassiterite 

SnC-2 

T 

(100) 

Pyramidal 

6.9 

6.5 

Red-brown 

Prismatic 

to  colorlei-= 

230 

Wurtzite 

ZnS 

H 

lolo 

Fibrous 

3.98 

3.5 

Yellow 
Brownish 

1  1  n 

230 

Zircon 

ZrSiC-4 

T 

(100) 

Prismatic 

4.7 

7.5 

Colorles 

231 

Xenotime 

Y2Os  phosphate 
containing  SOs 

T 

110 

Pyramidal 
Prismatic 

4.6 

6.5 

Colorles 
Light 
reddish 

232 

Corundum 

A12O3 

H 

Granular 
Pyramidal 

4.0 

9 

Colorles* 
Blue 

Tabular 

233 

Vesuvianite 

Calcium-aluminium 
silicate 

T 

Short 
prismatic 
Granular 

3.45 

6.5 

Colorless 

(Rose) 

Thick 

o. 

Gehlenite 

Ca3Al2Si2Oio 

tabular 

3.0 

2 

Isometric 

o 

Colorle-1 

233 

.-§ 

T 

001 

5.5 

(Yellowis: 

1 

Melilite 

Ca4Si3Oio 

Thin 
tabular 

2.9 

Precious 

Blue 

<u 

Tourmaline 

3  0 

Green  j 

234 

"3 

Aluminium  silicate 
containing  B2Os 

H 

Prismatic 
Acicular 

to 
3.25 

7.5 

Yellowis 
Brown 
Violet, 

Schorl 

** 

H 

DESCRIPTIVE  SECTION 


MINERALS. 


351 
TABLE  4 


a 

r 

r-a 

Chm 

Chz 

Solubility 

Remarks 

Minerals 

<   2.616 

2.903 

0.287 

+ 

+ 

Insoluble 

Adamantine     luster 
in  reflected  light 
Frequently  twinned 
and  laminated 
Absorption  £  >  w 

Rutile 

2.493 

2.554 

0.061 

- 

(  +  ) 

Insoluble 

Often  adamantine 
luster   in    reflected 
light 
Mottled  color 
Absorption  w  >  e 

Anatase 

1.997 

2.093 

;  0.096 

-f 

+ 

Insoluble 

Adamantine     luster 
in  reflected  light 
Zonal  structure 
Absorption  £>w 

Cassiterite 

>1 

93 

Low 

+ 

+ 

Easily  soluble  in 
cold  hydrochlo- 
ric acid 

Absorption  e  >  o> 

Wurtzite 

1.931 

1.993 

0.062 

+ 

+ 

Insoluble 

Sp.    gr.     =     4.0    in 
some  varieties 
Not  decomposed  in 
a  bead  of  microcos- 
mic  salt 

Zircon 

1.721 

1.816 

0.095 

+ 

+ 

Insoluble 

Often  decomposed  in 
the  rocks,  then  H 
=  4-5 
Clouded 

Xenotime 

1.760 
Light 
bluish- 
green 

1.769 
Blue 

0.009 

- 

(  +  ) 

Insoluble 

Mottled  color 
Absorption  w>£ 
Often    zonal    struc- 
ture 

Corundum 

1.701 

1.705 

0.001 

- 

- 

Difficultly  solu- 

Often  anomalous  in- 

1.726 

1.732 

0.006 

+ 

+ 

chloric  acid 

Optical  anomalies 

Vesuvianite 

1.657 

1.665 

0.008 

- 

Form   gelatinous 

Anomalous 
interference 
color 

Gehlenite 

- 

+ 

silica    with    hy- 
drochloric acid 

^\. 

1.629 

1.631 

0.003 

+ 

- 

Peg  structure  ^ 
When  colored  £  >  co 

Melilite 

1.620 

1.640 

0.017 

Weak  absorption 

W>£ 

Precious 
Tourmaline 

i     to 
1.651 

to 
1.685 

to 
0.034 

Insoluble 

Strong  absorption 

W>£ 

Pleochroic  halos 

Schorl 

352                       PETROGRAPHIC  METHODS 
UNIAXIAL 

Page 

a 
O 

Minerals 

Chemical 
composition 

Crystal 
system 

Cleav- 
age 

Develop- 
ment 

Sp.  gr. 

H 

Color 

236 

Apatite 

Ca6P»Oi2 
(C1.F) 

H 

Long 
prismatic 
Grains 

3.16 

5 

Colorless 
(Brown, 
yellow) 

238 

Rhombohedral  Carbonates 

Calcite 

CaCOj 

H 

1011 

Granular 
Radial 

2.72 

3 

Colorles 

Dolomite 

CaMg(C03)2 

H 

Rhombo- 
hedral 
Granular 

2.95 

4 

Magnesite 

MgCOs 

H 

3.0 

4.5 

Siderite 

FeCOs 

H 

Granular 
Rhombo- 
hedral 

3.9 

4 

ColorleM 
Yellowkl 

242 

Eudialyte 
(Eucolite) 

Silicate  containing 
ZrO2  and  Ce2Os 

H 

(0001) 

Grains 

3.0 

5.5 
5.5 

Colorlass 
(Reddisl 

242 

Scapolite  Group 

Marialite 

Na4Ai3Si9024Cl 

T 

110 

Prismatic 
Granular 
Columnar 

2.55 
to 
2.75 

Colorles, 

Meionite 

T 

243 

Alunite 

KAl3(OH)6(SO4)2 

H 

0001 

Rhombo- 
hedral 

2.7 

4 

Colorle 

243 

Beryl 

BesAl2Si6Oi8 

H 

(0001) 

Prismatic 

2.7 

8 

Colorle 
(Bluish 

244 

Brucite 

Mg(OH)2 

H 

0001 

Tabular 

2.35 

2.5 

Colorlw 

244 

1 

Quartz 

SiO2 

H 

Granular 
Pyramidal 

2.65 

7 

Chalcedony 

' 

Fine 
fibrous 

2.60 

6.5 

Colorles 

Tridymite 

H 

Tabular 

2.3 

DESCRIPTIVE  SECTION  353 

MINERALS.— Confirmed.  TABLE  5 


a 

r 

f-  a 

Chm 

Chz 

Solubility 

Remarks 

Minerals 

1.634 

1.637 

0.003 

- 

- 

Easily  soluble 

Absorption   when 
colored  e>o> 

Apatite 

1.487 

1.659 

0.172 

- 

Effervesces  in 
cold  acid 

Twinning  lamellae 
parallel     to     —  JR 
frequent 
(Absorption  u  >  £) 

Calcite 

1.503 

1.682 

0.179 

- 

(Absorption    w>e) 
Index  and  double  re- 

Dolomite 

1.515 

1.717 

0.202 

- 

Effervesce   in 
warm  acid 

fraction   increase 
with  the  content  of 
iron 

Magnesite 

1.633 

1.872 

0.239 

- 

Frequently  altered  to 
limonite 

Siderite 

1.608 
(1.617) 

1.610 
(1.620) 

0.002 
(0.003) 

<-> 

Gelatinizes  with 
hydrochloric 
acid 

Absorption    when 
colored  w>e 
Easily  fusible 

Eudialyte 
(Eucolite) 

1.542 

1.555 

0.013 

Not  attacked  by 
hydrochloric 
acid 

Precipitate  of  silver 
chloride     when 

Marialite 

1.558 

1.597 

0.039 

Easily  decom- 
posed by  hydro- 
chloric acid 

treated    with    HF 
and  AgNOs 

Meionite 

1.572 

1.592 

0.020 

+ 

Difficultly  solu- 
ble in  sulphuric 
acid 

Soluble     in      water 
after    ignition 

Alunite 

1.572 

1.577 

0.005 

- 

- 

Insoluble 

Often  apparently 
biaxial 

Beryl 

1.560 

1.581 

0.021 

+ 

-. 

Easily  soluble 

Anomalous  interfer- 
ence colors 
Brown  with  AgNOs 

Brucite 

1.544 

1.553 

0.009 

- 

+ 

Frequently    c  a  t  a- 
clastic 

Quartz 

1.532 

1.543 

0.011 

- 

- 

Entirely  soluble 
in    hydrofluoric 

Generally  apparent- 
ly biaxial  2V  up  to 
40° 

Chalcedony 

1.476 

1.478 

0.002 

+ 

- 

Frequently   twins 
Anomalous  interfer- 
ence, segments 
2E  about  65° 

Tridymite 

23 


354                       PETROGRAPHIC  METHODS 
UNIAXIAL 

Page 

1 

O 

Minerals 

Chemical 
composition 

Crystal 
system 

Cleav- 
age 

(1010) 
(0001) 

Develop- 
ment 

Sp.  gr. 

H 

Colo 

249 

Nepheline 

NaAlSiO4 

H 

Short 
prismatic 
Granular 

2.60 

6 

Colorle 

250 

Apophyllite 

Ca,  K  silicate  con- 
taining water 

T 

001 

Scaly 

2.35 

5 

Colorle 

250 

Chabazite 

CaAl2Si4O12  +  6aq. 
Contains  Na 

H 

loll 

Rhombo- 
hedral 

2.1 

4 

Colorle 

250 

Cancrinite 

NaAlSiO4  +  CO2 
and  aq. 

H 

loTo 

(0001) 

Columnar 

2.45 

•5.5 

Colorless  I 

251 

Hydro- 
nephelite 
(Ranite) 

HNa2Al3Si3Oi2  + 
3  aq. 

H 

dolo) 

Confused 
columnar 

2.25 

5 

Colorless 

DESCRIPTIVE  SECTION  355 

MINERALS.— Confirmed.  TABLE  6 


a 

r 

r-a 

Chm 

Chz 

Solubility 

Remarks 

Minerals 

1.538 

1.542 

0.004 

'  - 

Gelatinizes  easily 
with  hydrochlo- 
ric acid 

Frequently     altered 
Optical  anomalies 

Nepheline 

1.535 

1  .  537 

0.002 

~ 

(-) 

(  +  ) 

Forms  powdered 
silica  when  treat- 
ed with  hydro- 
chloric acid 

Cloudy    after    igni- 
tion, Anomalous  in- 
terference    colors, 
Segments 

Apophyllite 

About 

1.50 

Low 

(+> 

Gelatinizes  with 
hydrochloric 
acid 

Often  anomalous  in- 
terference        Seg- 
ments 

Chabazite 

1.496 

1.522 

0.025 

- 

- 

Gelatinizes  with 
weak    efferves-: 
cence  in  hydro- 
chloric acid 

Cloudy    after    igni- 
tion 
Frequently  altered 
Pseudomorph    after 
nepheline 

Cancrinite 

1.484 

1.496 

0.012 

•  + 

+ 

Gelatinizes  with 
hydrochloric  acid 

Pseudomorph    after 
nepheline 
(Spreustein) 

Hydronephe- 
lite 
(Ranite) 

356 


PETROGRAPHIC  METHODS 


BIAXIAL 


Page 

O 

Minerals 

Chemical 
composition 

Crys- 
tal 

sys- 
tem 

Cleav- 
age 

Develop- 
ment 

Sp. 
gr. 

H 

Color 

a 

0 

254 

Brookite 

TiOi 

0 

010 

Tabular  || 
{100} 

4.0 

6 

Brownish- 
red 
Yellowish 

(c>B=o) 

2.583 

2.586 

254 

Goethite 

FeO2H 

O 

010 

Fine  acicu- 
lar||{010} 

4.4 

5 

.  Brown 
Yellowish 

Above 

255 

Pseudo- 
brookite 

— 

o 

(010) 

Tabular  || 
{100} 

5.0 

6 

Brownish- 
red 
Hardly 
transparent 

Very 
high 

255 

Sulphur 

•     S 

o 

Pyramidal 
Granular 

2.05 

2 

Yellowish 

1.950 

2.038 

255 

Baddeley- 
ite 

Zr02 

M 

001 

Prismatic 
along  the 
baxis 

6.0 

6.5 

Greenish  to 
brown 

a=c>B 

High 

255 

Titanite 

Ca(SiTi)O6 

M 

110 

(134°) 

Flat  pris- 
matic |j 
{123}    or 
{110} 
Grains 

3.5 

6 

Colorless 
Yellowish 
Reddish 

(  t>  B  >o) 

1.888 
to 
1.913 

1.894 
to 
1.921 

256 

Lievrite 

HCaFe2Fe 
Si2Q9 

0 

(010) 

Prismatic 
Columnar 
Fibrous 

4.0 

6 

Usually  trans- 
parent paral- 
lel to  a  only 
Brownish- 
green 

.    About 

257 

Monazite 

CePO4 

M 

001 

Thick  tab- 
ular || 
{100} 
Grains 

5.15 

5.5 

Colorless 
Yellowish 
Brownish 

1.796 

1.797 

257 

Lavenite 

Silicate 
containing 
ZrC-2,  Ti02 
Nb206 

M 

(100) 

Prismatic 
||  c 
Tabular  || 
{100} 

3.5 

6 

Yellow  to 
colorless 

Bright 
yellow 

About 
1.75 
Yellow- 
ish- 
green 

257 

Chryso- 
beryl 

BeAl2O4 

0 

(010) 

Isometric 
grains 

3.73 

8.5 

Light  green 
Colorless 

1.747 
Reddish 

1.748 
Yellow- 
ish 

DESCRIPTIVE  SECTION 


MINERALS. 


357 
TABLE  7 


r 

r-a 

Chm 

Chz 

2V 

2E 

Disper- 
sion 

Optical  orien- 
tation 

Solubility 

Remarks 

Minerals 

2.741 

0.158 

+ 

+ 

Very 
small 

Crossed 
optic 
planes 

a=  C 

b=  aQ0) 
b-B(w) 

Insoluble 

Pleochroism 
Weak  adaman- 
tine luster 

Brookite 

2.5 

High 

- 

Med- 
ium 

Crossed 
optic 
planes 

b=a 

c=  t(p) 
a=  C(y) 

DiflBcultly 
soluble 

Pleochroism 
brown  to  yel- 
low     Ada- 
mantine lus- 
ter 

Goethite 

High 

+ 

- 

Large 

u>p 

a=  t 
b=0 

Soluble  in 
hot  H2SO4 

Adamantine 
luster 

Pseudo- 
brookite 

2.240 

0.290 

- 

70° 

a=0 
b=B 

Soluble  in 
KOH 

Opaque  to 
Roentgen  rays 

Sulphur 

High 

- 

+ 

70° 

c:0  =  13°f 
b=B 

Insoluble 

Always 
twinned 

Baddeley- 
ite 

1.978 
to 
2.054 

0.090 
to 
0.141 

.   + 

45° 
to 
60° 

p^o 

c:t  =  51°f 
b=B 

Soluble  in 
hot  H2SO4 

Twins       . 
Insect's  eggs 

Titanite 

1.89 

Un- 
known 

+ 

+ 

Gelatin- 
izes with 
HC1 

Easily  fusible 
to  black  mag- 
netic glass 

Lievrite 

1.841 

0.045 

+ 

+ 

25° 

u>p 

c:C  =  4°  f 
b-f 

White 
residue 
with  HC1 

Spectroscopic 
test 

Monazite 

Orange 

About 
0.03 

- 

- 

80° 

c:0  =  20°r 

b=B 

Attacked 
with  diffi- 
culty 

Easily  fusible 
Twins 

Lavenite 

1.756 
Green 

0.009 

+ 

0° 
to 
90° 

P>1> 

u>/9 

Insoluble 

Dispersion  ex- 
tremely vari- 
able 

Chryso- 
beryl 

358 


PETROGRAPHIC  METHODS 


BIAXIAL 


ft 

Crys- 

Page 

E 

o 

Minerals 

Chemical 
composition 

tal 

sys- 

Cleav- 
age 

Develop- 
ment 

Sp. 
gr. 

H 

Color 

ft 

tem 

Zoisite  ct 

1.697        1.699 

100 

o 



3  3 

Zoisite  /? 

HCa2Al3Si3Ois 

and 
(001) 

Colorless 
Light  red- 

About 
1.7 

Clinozois- 

3  35 

ite 

o, 

1.718        1.720 

258 

I 

8 

Epidote 

(Pistazite) 

HCa2(AlFe)3 
Si3Oi3 

Prismatic 
along  the 
b  axis 

7 

Yellowish 
Greenish 

1.731         1.754 

001 

Granular 

C 

w 

M 

and 

3.4 

Red 

... 

Piemont- 
ite 

HCa2(AlFeMn)3 
SisOis 

(100) 
115i° 

Violet 
Yellow 

Orange       Violet 

c<  fe>a 

Brown 

Orthite 

HCa2(AlFeCe>» 

3.6 

(Pale   red- 

About 

(Allanite) 

SisOi3 

3.8 

dish) 

1.78 

266 

Staurolite 

HFeAl5Si2Oi3 

0 

(010) 

Prismatic 
Prism    angle 

3.6 
to 

7.5 

Yellow,  Red- 
dish brown 

• 
1.736          1.741 

129° 

3.8 

c>B  =  o 

267 

Diaspore 

A1O2H 

,>;  , 

0 

010 

Platy  || 
{010} 

3.5 

6 

Colorless 
(Bluish) 

1  .  702        1  .  722 

100 

Prismatic 

' 

267 

Cyanite 

Al2SiO5 

Tr 

(010) 
001 

lie 
Tabular   || 
{100} 

3.6 

4-7 

Colorless 
Bluish 

1.712    |    1.720 
Color-    |     Pale 
less          violet 

Tabular  || 

Very  pale 

1.706 

268 

Sapphirine 

Mg5Ah2Si2O27 

M 

{010} 

3.5 

7.5 

blue  to  blu- 

Color-       1JS 

Granular 

ish  green 

less 

Aluminium 

269 

Serendi- 
bite 

silicate    con- 
taining B2Os 

Tr 

Plates 
Grains 

3.4 

7 

Blue 
Pleochroic 

About 
1.7 

(CaMgFe)O 

269 

Prismatine 

MgAl2SiO6 

O 

100 

(81°) 

Prismatic 
Columnar 
Radial 

3.3 

6.5 

Colorless 
Yellowish 

1.669         1.680 

DESCRIPTIVE  SECTION 


— Continued. 


359 
TABLE  8 


r 

f-a 

Chm 

Chz 

2V 

2E 

Disper- 
sion 

Optical  orien- 
tation 

Solubility 

Remarks 

Minerals 

1.702          0.005 

-f 

- 

90° 

*j 

c=  C 
b=0 

Anomalous 
interference 

Zoisite  a 

Very 
low 

- 

* 

50° 

p>u 

c=  C 

b=B 

twinning 
lamination 

Zoisite  /? 

+ 

80° 

u^p 

c:0  =  2°  f 

Gelatinize 

\ 

Clinozois- 

\     1.723          0.005 

C:a  about  30° 

in  HC1 

\ 

ite 

to 

to 

after 

\ 

1.768 

0.061 

- 

+ 

70° 

p>u 

c:0  =  3°r 

b=B 

C:a  about  30° 

melting 

\ 

\ 
\ 

Epidote 

(Pistazite) 

Carmine 

I 

* 

Vari- 
able 

c:  0  =  2°  to  7°  r 
C  :a  about  30° 

Speckled  \ 
interfer-     \ 
ence  colors  \ 

Piemont- 
ite 

0.002 
to 
0.030 

- 

t 

70° 

c:0  =  36°r 

b=B 

0:a  about  30° 

Difficultly 
soluble  in 
HCJ 

Surrounded 
by  pleochroic 
halo 

Orthite 
(Allanite) 

1.746 

0.010 

^ 

- 

85° 
to 
90° 

Weak 
p>u 

c=  C 

b=0 

Insoluble 

Pleochroic 
halos 
Twins 

Staurolite 

1.750 

0.048 

+ 

- 

85° 

u>f 

a=  C 

b=B 

Insoluble 

Diaspore 

Twinning  j| 

1.728 
Pale 
blue 

0.012 

to 
0.016 

- 

- 

82° 

p>u 

0  about  1  {100} 
c:C=+30° 

Insoluble 

{100} 
Fibrous  frac- 
ture || 

Cyanite 

{001} 

1.711 
light  blue 

0.005 

- 

69° 

,,, 

c:a  =  81°f 

Insoluble 

Infusible 

Sapphirine 

0.006 

Large 

c:0  about  40° 

Insoluble 

Always  twin 
lamellae 

Serendi- 

Infusible 

1    1.682 

0.013 

- 

- 

65° 

p>u 

c  =   0 

b  =   C 

Insoluble 

Infusible 

Prismatine 

360 


PETROGRAPHIC  METHODS 


BIAXIAL 


a 

Crys- 

Page 

2 

o 

Minerals 

Chemical 
composition 

tal 

sys- 

Cleav- 
age 

Develop- 
ment 

Sp. 
gr. 

H 

Color 

a 

f 

tem 

269 

Astrophyl- 
lite 

Alkali  iron 
silicate  con- 
taining TiO2 

O 

100 

Tabular- 
Prismatic 

ilb 

3.3 

3.5 

Orange 
Brownish 
Red 
tt>fi>C 

1.678 
Deep 
orange 

Orange 

269 

i 

a 

Chloritoid 

Basic  alumi- 
nium silicate 
containing 
(FeMgCa)O 

Tr 

001 
(110) 

Tabular  || 
{001} 
Platy 

3.5 

6.5 

Bluish-green 
Colorless 

1)  >  a  >  c 

Olive- 
green 

1.741 
Indigo 

Xantho- 
phyllite 

M 

3.1 

5 

Colorless 
Light  green 

1.649 

1.66" 

272 

Margarite 

H2CaAU 

Si2Oi2 

1C 

001 

Tabular 
Flaky 

3.0 

4 

Colorless 

1.66, 

Forsterite 

Mg2Si04 

1.657, 

272 

I 

Olivine 

(MgFe)2Si04 

Q 

010 

Short  pris- 
matic ||  c 

3.2 
to 
3.6 

7 

Colorless 

1.661 

1.678 

Oii  vine 

Fayalite 

FesSiOi 

(100) 

Grains 

6.5 

Yellow 
Brownish 

1.824 

1.86 

Monticel- 
lite 

CaMgSiO4 

3.2 

5.5 

Colorless 

1.651 

1.66, 

DESCRIPTIVE  SECTION 


MINERALS.— Continued. 


361 
TABLE  9 


r 

r-a 

Chm 

Chz 

2V 

2E 

Disper- 
sion 

Optical  orien- 
tation 

Solubility 

Remarks 

Minerals 

1.733 
Light 
yellow 

0.055 

- 

+ 

Very 
large 

p>u 

c=B 

b=  C 

Difficultly 
soluble 

Easily  fusible 
Star-shaped 
aggregates 

Astro- 
phyllite 

Greenish- 
yellow 

0.003 
to 
0.015 

+ 

- 

65° 
to 
120° 

„, 

c:C  =  0°-15° 
b:tt  =  0°-25° 

Insoluble 

Twinning 
after  the  mica 
law 
Scarcely 
fusible 

Chlori- 
toid 

1.661 

0.012 

- 

+ 

0° 

to 
25° 

u>p 

c=0 

Xantho- 
phyllite 

0.009 

- 

- 

Large 

b-C 

Quite 
difficultly 
soluble 

Twinning 
lamellae 

Margarite 

1.697 

About 
0.03 

+ 

+ 

85° 

Weak 

a=  C 
b=0 

Gelatinize 
slowly 
with  cold 
HC1 

Alteration 
into 
serpentine 

Forsterite 

0.036 

(-> 

About 
90° 

Distinct 
u>p 

Olivine 

1.874 

0.050 

- 

50° 

p>u 

Fayalite 

Monticel- 
lite 

1.668 

0.017 

38° 

,>. 

362 


PETROGRAPHIC  METHODS 


BIAXIAL 


Crys-  • 

Page 

3 

o 

Minerals 

Chemical 
composition 

tal 

sys- 

Cleav- 
age 

Develop- 
ment 

Sp. 
gr. 

^ 

Color 

a. 

ft 

tem 

Enstatite 
(Bronzite) 

MgSiOs 

110 

Colorless 

1.660 

1.665 
to 

O 

100 
(010) 

3.1 
to 
3.5 

to 
1.716 
Red- 
brown 

1.725 
Yellow 
ish- 
brown 

Hyper- 
sthene 

(MgFe)SiOs 

Brown 

'          1 

Diopside 

(MgCaFe)SiO3 

5.5 

Colorless 
Green 
Scarcely 
pleochroic 

1.671 
to 
1.699 

1.678 
to 
1.706 

a 

Diallage 

Short  pris- 

3.3 

Greenish 
Brownish 

1.679 
Green- 
ish 

1.681 
Yellow- 
ish 

P 

matic  along 

277 

0 
a> 

Fassaite 

the  c  axis 

Green 

Green 

Yellow- 

i 

green 

* 

Augite 

(MgFe)  (AlFeh 
SiO6 

M 

110 

87° 

Greenish 
Brownish 

1.698 
to 
1.706 

1.704 
to 
1  .712 

Green 

Aegirine- 
augite 

3.4 

Violet  with 
content  of 

Green 

Bright 
green 

to 
3.5 

TiO2 

6 

1   799 

Aegirine 

(Acmite) 

NaFeSi2O6 

Sap  green 
(Brown) 

1.763 

Grass- 
green 

(Brown) 

Light 
green 

(Light    ! 
brown) 

Spodu- 

LiAlSi2O6 

Prismatic 
to  tabular 

3.1 

6.5 

Colorless 

1.660 

1.666 

mene 

||  {100} 

Tabular  || 

286 

Lawsonite 

H4CaAl2Si2Oio 

O 

{001} 
Prismatic 

3.1 

8.5 

Colorless 

1.665 

1.669 

001 

lie 

DESCRIPTIVE  SECTION                         363 

MINERALS.—  Contained.                                                                                 TABLE    10 

r 

r-a 

Chm 

Chz 

2V 

2E 

Disper- 
sion 

Optical 
orientation 

Solu- 
bility 

Remarks 

Minerals 

77° 
to 

\^      Infusible 

Ensta- 
tite 

to 

0.010 

90° 

For 

(Bronzite) 

1.729 

to 

FeO  =  10  %    .; 

Gray- 
green 

0.013 

90° 
to 
50° 

p>u 

b=  a 

Xficultly 

Hyper- 
sthene 

XX      fusible 

1   700 

to 
1  .  727 

0.029 

KOO 

1  OQ° 

p>u 

c:C  =  39°-44°f 

Difficultly         Diopside 
fusible 

Twinning 

1  .  703 
j  Greenish 

0.024 

c:C  =  40°f 

lamellae  and 
parting  i|  {100} 

Diallage 

and  {001} 

\     Green 

0.021 

c:C  =  45°f 

b=B 

Difficultly 
soluble 
even  in 

Strong  dis- 
persion of  the 
bisectrices 

Fassaite 

±1*       i 

1.723 
to 

0.022 
to 

Vari- 
able 

c:C  =  54°  f 

Frequently  a 
few  twinning 

Augite 

1.728 

0.025 

lamellse 

Greenish- 

c:C  =  60°  f 

Aegirine- 

yellow 

b=B 

Strong  dis- 

augite 

persion  of  the 

1.813 

Yellow- 
ish 
(Green- 

0.050 

- 

- 

64° 

c:C  =  94°f 

Easily 
fusible 

Aegirine 

(Acmite) 

ish) 

I 

1.676 

0.016          + 

+ 

60° 

u>p 

c:C  =  25° 

Easily  fusible 
with  intumes- 

Spodu- 
mene 

cence 

Gelatin- 

1.684 

0.019 

t 

84° 

p>u 

c=  C 

b=B 

izes  with      Easily  fusible 
HC1  after            Twins 

Lawsonite 

melting 

364 


PETROGRAPHIC  METHODS 


BIAXIAL 


o. 

Crys- 

Page 

2 

O 

Minerals 

Chemical 
composition 

tal 
sys- 

Cleav- 
age 

Develop- 
ment 

Sp. 
gr. 

H 

Color 

a. 

tem 

Antho- 
phyllite 
(Gedrite) 

(MgFe)- 
SiOs  con- 
taining 
A12O3 

O 

100 
110 

(010) 

Columnar 
{110}  =124° 
Flaky 
Fascicular 

3.1 

5.5 

Colorless 
Light  brown- 
ish 
Reddish 
(  C>  fc  >  0) 

1.633 

1.64 

Actinolite 

(Tremolite) 

(MgFeCa) 
SiOa 

3.0 

Colorless 
Light  green 

1.607 

Pargasite 

(MgFeCa) 

Light  green 
(Brownish) 

1.616 

1.62i 

AloSiOa 

Green 
hornblende 

3.1 

to 

Green,  Bluish- 
green,  Brown- 
ish-green 

1.629 
Yel- 
lowish 

a 

lowisf 

3.2 

c>b  >  o 

greet 

I 

Brown 

(MgFeCa) 
(AlFe)2- 

§ 

Prismatic 
along  the 

Brown 

Yel- 

1.6-1 

i 

hornblende 

SiO6 

•£ 

c  axis 

5.5 

c=B>a 

lowish 

Brawii 

r\ 

to 

287 

•I 

M 

6 

1 

Basaltic 
hornblende 

Contains 
TiO2 

Containir 

M 

110 
124° 
(001) 

3.3 

Red  brown 
Yellowish- 
brown 
Deep  brown 

1.680 
Yel- 
•  lowish- 
brown 

Brov>; 

1.621 

Glauco- 

phane 

NaAl- 
Si206 

3.1 

Light  blue 

Light 
yellow- 

Violo 

ish 

Crocido- 
lite 

Fibrous 
Resembles 

Light  blue 

Blue 

Viol* 

asbestos 

Riebeckite 

NaFe- 
Si20e 

Short  pris- 
matic 
Irregular 
shreds 

3.4 

5 

Deep  blue 

Black- 
ish- 
blue 

Aboi 
1.7 
Blue 

Arfved- 

3.5 

Bluish-green 

Green- 
ish- 

Abo  a 
1.7 

blue 

Bluet, 

295 

Dumor- 
tierite 

AUSisOig  con- 
taining boron 

O 

100 

Acicular  ||  c 

3.3 

7 

Violet 
Blue 

1.678 
Bluish- 
violet 

1.68 

Yel- 
lowisi 

DESCRIPTIVE  SECTION 


MINERALS.— Continued. 


365 
TABLE  11 


r 

r-a 

Chm 

Chz 

2V 

2E 

Disper- 
sion 

Optical 
orientation 

Solu- 
bility 

Remarks 

Minerals 

1.657 

0.024 
Varia- 
ble 

i 

Very 
large 

u>p 

(p>U) 

c=  t 

Pleochroic 
halos 

Antho- 
phyllite 
(Gedrite) 

1.634 

0.027 

80° 

u>p 

c:C  =  10°-20°f 
b      fi 

Very  weakly 
pleochroic  at 

Actinolite 
(Tremo- 

best 

lite) 

1.635 

0.019 

- 

55° 

97° 

p>u 

c:C  =  18°f 

b=B 

Short  thick 
prisms 

Pargasite 

1.653 

Blue- 
brown 
Deep 

0.024 

84° 

c:C  =  12°-20°f 

Green 
hornblende 

green 

Pleochroic 

Brown 

About 
0.025 

c:C  =  14°f 

Attacked 

Brown 
hornblende 



with  dif- 

I  752 

Magmatic 

Deep 
arown 

0.072 

80° 

c:C  =  10°-0°f 

resorption 
Strong  dis- 
persion 

Basaltic 
hornblende 

;1.639 

ky  blue 

0.018 

50° 

85° 

c:C  =  4°-6°f 

Glauco- 
phane 

Llmost 
color- 
less 

0.025 

+ 

95° 

c:0  =  18°-20°f 

b=B 

Easily  fus- 
ible 

Crocido- 
lite 

Bluish- 
green 

0.003 

- 

- 

Very 
large 

I)  ^>  p 

c:0  =  5°r 

b=B 

Strong  dis- 
persion of  the 

Riebeckite 

K4  0  A/t-f  f.1  r»f\& 

Irayish- 
green 

Low 

' 

Very 
large 

c:0  =  14°r 

b=B 

Very  easily 
fusible 

Arfved- 
sonite 

1.689 

Decolorized 

Color- 
less 

0.011 

- 

- 

30° 

54° 

p^o 

b=B 

Insoluble 

by  strong  ig- 
nition 

Dumor- 

tierite 

Infusible 

. 

366                       PETROGRAPHIC  METHODS 

BIAXIAL 

Crys- 

Page 

a 

S 

o 

Minerals 

Chemical 
composition 

tal 

sys- 

Cleav- 
age 

Develop- 
ment 

Sp'      H            Color                 a 
gr. 

tem 

295 

Axinite 

Aluminium 
silicate  con- 
taining boron 

Tr 

(110) 

Wedge  shape 
Granular 

3.3 

Colorless          1.672 
Light  violet     Colorless 

i 

Cerium  silicate 

i  nn 

Thick  tabu- 

Colorless 

Rinkite 

containing 

M 

lar  i| 

3.46 

5        Yellowish         1.665        1. 

295 

TiC-2  and  Fl 

{100} 

c=fe>a 

295 

Sillimanite 

Al2SiO5 

O 

100 

Acicular 

3.24 

6.5        Colorless      !     1.656        1. 

II  c 

296 

Datolite 

HCaBSiOs 

M 

Granular 
Fibrous  ||  b 

3.0 

5         Colorless          1.626        1. 

297 

Mosan- 
drite 

Similar  to 
rinkite.  Con- 
tains ZrO2 

M 

100 

Tabular  || 
{100} 

3.1 

1.646        1.61 
Colorless         ,„ 
4       /-IT-  11      •  u\    i  (Green-    (Brov 
(Yellowish) 
ish)            isli 

oni 

Granular 

297 

Barite 

BaSC-4 

O 

Platy  || 

4.5    3.5        Colorless          1.637        !.(> 

110 

{001} 

" 

297 

Andalusite 

Al2Si05 

0 

110 
91° 

Prismatic 

lie 

3.2 

Colorless          1.622         l.t 
Reddish           Rose        Cok 

298 

Lazulite 

H2(FeMg) 
A12P2O9 

M 

Pyramidal 

3.0 

1.603              . 
5             Blue             Color-        "j 

less 

299 

Carpholite 

H4MnAl2Si2Oio 

M 

(010) 

Acicular 
Fine  fibrous 

2.9 

5         Yellow 
\  ellowist 

i 

Tabular  || 

j                            i 

299 

Prehnite 

H2Ca2Al2 
SisC-12 

O 

001 
(001) 

{001} 
Fibrous 

2.9 

1.616         l.i 
6.5        Colorless                    y^y 

Rosettes 

299 

Celestite 

SrSO4 

0 

001 
(110) 

Granular 
Fibrous 

3.95 

3.5        Colorless          1.622         l.»: 

DESCRIPTIVE  SECTION 


367 
TABLE  12 


r         r-« 

Chm     Chz 

2V 

2E 

Disper- 
sion 

Optical                Solu- 
orientation             bility 

Remarks 

Minerals 

1.681 
Color- 

0.000 

_ 

72° 

x 

u>p 

Insoluble 

Test  for 
boron 

i     Axinite 

less 

Twinning 

1.681 

0.016 

+ 

+ 

80° 

»>P 

c  :  b  =  7°  r           Easily  de-       lamellae  || 
b=a              composed            {100} 

Rinkite 

Fuses  easily 

1.677 

0.021          + 

+ 

20°  to 
30° 

35°  to 
55° 

P>u 

c=  C                                      Often    very 
i   Insoluble       «        -i_  . 
b=  0                                     fine   fibrous 

Sillimanite 

1.670 

0.044 

_ 

74° 

p>u 

Gelatinizes         „     ,  , 
c:0  =  4°r                                     Test  for 
,       *.             readily  in              , 
b  =  0                    TT«,                  boron 

Datolite 

HC1 

1.658 

(Light 
yellow) 

0.012 

- 

70° 

„, 

Twinning 
c:0  =  2°           Easily  de-        lamellae  || 
b=B              composed            {100} 
Fuses  easily 

.Mosan- 
drite 

Soluble  in 

1.649 

0.012          + 

37° 

63° 

u>p 

warm 

Barite 

H2SO4 

1.643 
less 

0.011 

- 

- 

84° 

Pleochroic 
Insoluble  j          halos 
Chiastolite 

Andalu- 
site 

1.639 

blue 

0.036 

- 

69° 

* 

Attacked 
~  «    T           with  dif- 
ficulty 

Decolorizes 
giving  off  wa- 
ter upon   igni- 
tion 

Lazulite 

Colorless      0.022 



+ 

60° 

Attacked 

°b-  o           *dth  dif" 

Carpho- 
lite 

ficulty 

1.649 
variable 

0.033 

- 

- 

Variable 

Gelatin- 
c  =  C               izes  with 
b=B              HC1  after 
melting 

Optical 
anomalies 
Parquet  forms 

Prehnite 

1.631 

0.009 

+ 

About 
Q0° 

Soluble 
c=  C 
fi                only  in 

Fusible 
Colors  flame 

Celestite 

hot  H2SO4 

red 

368 


PETROGRAPHIC  METHODS 


BIAXIAL 


Crys- 

Page 

| 

2 
o 

Minerals 

Chemical 
composition 

tal 

Sys- 

Cleav- 
age 

Develop- 
ment 

Sp. 
gr. 

H 

Color 

a 

tem 

299 

Aragonite 

CaCOs 

0 

Columnar 

lie 

2.95 

4 

Colorless 

1.530 

1.682 

Silicate  con- 

Rosen- 
buschite 

taining 
ZrOz,  TiO2 
LaaOs 

(100) 

Radial  || 
b  axis 

3.3 

5.5 

Yellowish 

Abou 
1.65 

3 

1 

001 

«> 

299 

*3 

WoUaston- 
ite 

CaSiOs 

M 

100 

(102) 

Columnar 
||  b  axis 

2.85 

5 

1.621 

1.63:5 

S3 

(101) 

!> 

Colorless 

Pectolite 

(CaNa2H2) 
SiOs 

100 
001 

Sheaf-like 

||  b  axis 

2.8 

4.5 

Abo 
1.6 

0. 

Humite 

Mg7(FOH)2 

Si30l2 

0 

300 

1 

-2 

Clino- 
humite 

Mg9(FOH)2 
Si4Oi6 

001 

Rounded 
grains 

3.1 

to 

39 

6.5 

Colorless 
Yellowish 

1.607 
to 

1.6H 
to 
1.67( 

0 

Yellow 

Color 

w 

Chondro- 

Mg5(FOH)2 

dite 

Si208 

301 

Topaz 

Al2SiO4 
(FOH)2 

O 

001 

Granular 
Columnar 

||  caxis 

3.4 
to 
3.6 

8 

Colorless 

1.607 
.      to 
1.629 

1.61 

to 
1.61 

DESCRIPTIVE  SECTION 


MINERALS.— Continued. 


369 
TABLE  13 


r 

f-Oi 

Chm 

Chz 

2V 

2E 

Disper- 
sion 

Optical 
orientation 

Solu- 
bility 

Remarks 

Minerals 

1.686 

0.156 

- 

- 

18° 

31° 

U>p 

c=0 

b=  t 

Readily 
soluble 
inHCl 

Aragonite 

About 
0.026 

- 

About 
90° 

c:C  =  13°r 
b=0 

Easily  fusible 

Rosen- 
buschite 

1.635 

0.014 

- 

! 

40° 

70° 

"- 

c:C  =  32°r 

b=B 

Readily 
soluble 
inHCl 

Powder  re-    / 
acts  alka-/ 
lino        / 

Wollaston- 
ite 

About 
0.038 

+ 

+ 

60° 

c:0  =  5°f 
b=C 

Twinning 
lamellas  par- 
allel to  {100} 

Pectolite 

c=B 

b=C 

Humite 

T      t         "IO                / 

1.639 
to 

0.032 

+ 

About 
70° 

Very 

c:|J=90f 
b=C 

Gelatin- 
ize readily 

•ran'+fi   TTP1 

Clinohum- 
ite 

less 

b=  C 

/     ning 
/    lamellae 

Chondro- 
dite 

1.618 

0.011 

67° 

126° 

Chz  formed 

to 

to 

+ 

+ 

to 

to 

p>o 

c=  t 

Insoluble 

by  cleavage, 

Topaz 

1.637 

0.008 

50° 

86° 

negative 

24 


370 


PETROGRAPHIC  METHODS 


BIAXIAL 


Crys- 

Page 

Minerals 

Chemical 
composition 

tal 

sys- 

Cleav- 
age 

Develop- 
ment 

Sp. 
gr. 

H 

Color 

a 

ft 

tem 

001 

301 

Anhydrite 

CaSC-4 

O 

010 

Granular 

2.95 

3 

Colorless 

1.570 

1.57; 

(100) 

Muscovite 

K2O 

Colorless 

1.562 

1  .  593 

"o 

2.8 

1 

to 
3  0 

Colorless 

a 

Phlogopite 

8       MgO 

•3 

Light 
brownish 

1.541 

1.573 

0 

t 

Tabular  || 

c=B>o 

302 

O 

1 

M 

001 

{001} 

2.5 

0 

| 

Scaly 

3.0 

Brown 

s 

Biotite 

FeO 

to 

Green 

1.580 

1.638 

| 

3.2 

C  ==  0  ^  0 

3 

Lithionite 

g       Li20 
3      (FeO) 

2.8 
to 

Colorless     . 
Brown 

1.562 

1.600 

3.2 

e-*>0 

Pennine 

H4(MgFe)2 
Al2SiO9 

M 

Tabular  || 
{001} 
Scaly  to 
radial 

2.6 
to 
3.0 

2.5 

Green,    ||   c 
light  yellow 
_L  c  green 
Colorless 

1.576 
to 
1.585 

1.576 
to 
1.58c 

001 

o. 

Clino- 

1 

chlore 

0 

310 

£ 

§ 

Antigorite 

Flaky 

1.560 

1.57C 

Colorless 

H4(MgFe)3 
Si2O9 

O? 

2.5 

to 

2  7 

3.5 

Light  green- 
ish 

Chrysotile 

110 
About 

Fibrous 

Rarely  green 

Aboi; 

•I       C  ". 

130° 

316 

Hydrar- 
gillite 

A1(OH)3 

M 

001 

Scaly 
Fibrous 

2.4 

2.5 

Colorless 

1.535 

,K 

DESCRIPTIVE  SECTION 


MINERALS.— Continued. 


371 
TABLE  14 


r 

r-a 

Chm 

Chz 

2V 

2E 

Disper- 
sion 

Optical 
orientation 

Solu- 
bility 

Remarks 

Minerals 

Difficultly 

Readily    al- 

1.614 

0.044 

+ 

43° 

71° 

U>p 

a—  C 

soluble  in 

tered  to  gyp- 

Anhydrite 

water 

sum 

30° 

55° 

Twin  lami- 

1.603 

0.041 

to 
50° 

to 
90° 

„>„ 

c=0 

b=C 

nation  paral- 
lel {001] 

Muscovite 

1.575 

0.034 

0° 
to 
25° 

c=  a 
b=B 

Attacked 
with  diffi- 
culty but 

observed 
Fuses  with 
difficulty 

Phlogopite 

more  easi- 

!   + 

ly  the 

V 

0° 

c:  0  =  0°  to  (10°) 

higher  the 

>v 

1.638 

0.058 

to 

b=  I) 

content  of 

\x 

Biotite     . 

70° 

(b=C) 

Mg  and 

>v 

(p>u) 

Fe 

Pleochroic    / 

halos     ,/ 

0° 

1.606 

0.044 

to 

p>u 

c=0 

b=  C 

/    Readily 

Lithionite 

90° 

fusible 

Very  variable 

optically 

i 



^ 

c—  C  (0) 

Anomalous 

(-) 

(  +  ) 

0° 

Oo>y) 

interference 

Pennine 

colors 

1.577 
to 

0.001 

^^^^ 

1.596 

0.011 

Pleochroic  / 

halos   / 

+ 

Variable 

„>„ 

c:C  =  0°-15°f 

Gelatin- 
ize with 
HCl 

/  Twin- 
/  ning  lam- 
/ellffi  ||  {001} 

Clinochlore 

1.571 

0.011 

- 

+ 

c=0 

b=C 

Pseudomorph 

Antigorite 

Very 

after  olivine 

weak 

Normal  inter- 

0.009 

+ 

+ 

Quite  small 

p>u 

c=  C 
b=tt 

ference  colors 
Very  diffi- 
cultly fusible 

Chrysotile 

1.510 

0.025 

+ 

- 

0° 
to 
40° 

p>u 

001:0  =  65°  f 

Soluble  in 
ho't  H2SO4 

Twinning 
lamellee 
II  {001} 

Hydrar- 
gillite 

Infusible 

372 


PETROGRAPHIC  METHODS 


BIAXIAL 


Crys- 

Page 

ft 

1 

O 

Minerals 

Chemical 
composition 

tal 

sys- 

Cleav- 
age 

Develop- 
ment 

Sp. 
gr. 

H 

Color 

a 

0 

tem 

316 

Talc 

H2Mg3Si4()i2 

M 

001 

Scaly 

2.7 

1 

Colorless 

1.539 

1.589 

317 

Pyrophyl- 
lite 

HAlSi2O8 

O 

001 

Radial 
Fibrous 

2.8 

1 

Colorless 

1.58 

317 

Bertrand- 
ite 

H2Be4Si2O9 

O 

010 
001 

Tabular  || 
{001}  or 

2.6 

6 

Colorless 

1.57 

{010} 

110 

318 

Wagnerite 

Mg2FPO4 

M 

Prismatic 

3.0 

5.5 

Colorless 

1.569 

1.570 

| 

Kaolin 

H4Al2Si2O9 

001 

2.5 

2.5 

Colorless 

1.55 

318 

e! 
£ 

3 

Nontro- 
nite 

H4Fe2Si2O» 

M 

001 

(110) 

Scaly 

2.7 

2 

Yellow  ' 

Light 
yellow 

About 
1.6 
Greenish- 

010 

319 

Hydro- 
magnesite 

Mg4(C03)3 
(OH)2  +  3aq. 

M 

100 

Columnar 

2.15 

2 

Colorless 

About 
1.54 

319 

Cordierite 

MgzAUSisOis 

O 

Granular 

2.65 

7 

Colorless 
Light  blue 

1.535 

Colorless 

1.540 
Blue 

320 

Wavellite 

AlsP2O8  +  12aq. 

0 

110 

Acicular 
Spherulitic 

2.4 

3.5 

Colorless 

1.526 

010 

321 

Gypsum 

CaSC-4  +  2aq. 

M 

(100) 

Granular 
Fibrous 

2.3 

2 

Colorless 

1.521 

1.523 

Ill- 

DESCRPITIVE  SECTION 


MINERALS—  Continued. 


373 
TABLE  15 


r 

r-« 

Chm 

Chz 

2V 

2E 

Disper- 
sion 

Optical 
orientation 

Solubility 

Remarks 

Minerals 

1.589 

0.050 

- 

4- 

0° 
to 
17° 

Small 

p>U 

c==0 

Attacked 
with 
difficulty 

Similar  to 
muscovite 
Distinguished 
by  cobalt 
solution 

Talc 

0.041 

- 

+ 

62° 

109° 

p>o 

c=  0 
b=C 

Soluble 
in  hot 
H2SO4 

Similar  to 
muscovite 
Distinguished 
by  hydrofluo- 
silicic  acid 

Pyrophyl- 
lite 

High 

- 

i 

75° 

u>p 

a=  n 

Insoluble 

Infusible 

Bertrand- 
ite 

1.582 

0.013 

* 

+ 

26° 

p>o 

c=  C 

b=B 

Easily 
soluble 

Very  diffi- 
cultly fusible 

Wagnerite 

0.008 

+ 

About 
90° 

p>u 

c:0  =  20°r 
b=  C 

Soluble  in 
hot  H2SO4 

Infusible 

Kaolin 

yellow 

0.02 

- 

+ 

60° 

c:C  =  6° 

Gelatin- 
izes with 
HC1 

Fusible  to 
black  slag 

Nontronite 

About 
0.02 

+ 

About 
120° 

c:0  =  33° 
b=  C 

Easily  sol- 
uble with 
efferves- 
cence 

Twinning 
lamination 
||  {100} 

Hydro- 

magnesite 

1.544 

Colorless 

0.009 

- 

Variable 

o>p 

c=  a 

b=C 

Attacked 
with  diffi- 
culty 

Penetration 
trilling 
Pleochroic 
halos 

Cordierite 

0.025 

+ 

«• 

50° 

p>u 

c=  C 
b=0 

Soluble 

Wavellite 

1.531 

0.010 

- 

60° 

104° 

Inclined 

c:C  =  54°f 

Difficultly 
soluble  in 
water 

{111)  Fi- 
brous fracture 
Twinning 
lamellae 

Gypsum 

374 


PETROGRAPHIC  METHODS 


BIAXIAL 

8  9         10 


Page 

1 

0 

Minerals 

Chemical 
composition 

Ab:An 

%Si02 

Sp.gr. 

Indices  com- 
pared with 
quartz 

a 

/? 

321 

a 

i 

Orthoclase 

(KNa)AlSi3O8 

65% 

2.56 

(w  for  quartz 
=  1  .  544) 

1.519 

1.524 

Microcline 

a] 
/?    <  a, 
f\ 

1.522 

1.526 

Anorthoclase 

(NaK)AlSi3Os 

2.58 

1.523 

1.529 

Albite 

NaAlSiaOs 
1 

t 
CaAl2Si2O8 

Ab 

68% 

2.62 

a] 

P  \<  a, 

r\ 

1.532 

1.534 

Albite- 
oligoclase 

AbeAm 

65% 

2.64 

1.534 

1.538 

Oligoclase 

Ab<Am 

64% 

)->w;  <x<  w 

1.537 

1.541 

Oligoclase- 
andesine 

Ab2Am 

60% 

2.65 

Y=£\  a  =  <i> 

1.544 

1.548 

Andesine 

AbsAn2 

58% 

2.67 

•jf>e;  a<  e 

1.549 

1.553 

Labradorite 

AbiAm 

55% 

2.69 

a] 
P\>9 

r\ 

1.555 

1.558 

Labradorite- 
bytownite 

Ab2Ans 

53% 

2.70 

1.560 

1.563 

Bytownite 

AbiAn4 

48% 

2.72 

1.564 

1.569 

Anorthite 

An 

44% 

2.75 

(s  for  quartz 
=  1.553) 

1.575 

1.583 

DESCRIPTIVE  SECTION 


MINERALS.— Continued. 
11          12          13       14          15  16 


17 


375 
TABLE  16 

18  19          20          21  22 


r 

r-a 

Chm 

2V 

Disper- 
sion 
about 

a 

Extinction  Angles 

Minerals 

on  001 

on  010 

JL  0 

_L  C 

_La 

Zone 
±010 

1.526 

0.007 

- 

69° 

p>0 

0° 

+  5° 

0° 

5° 

0° 

0° 

Orthoclase 

1.529 
1.530 

- 

88° 

p>o 

+  15i° 

+  5*° 

88° 

10° 

Microcline 

45° 

p>u 

+  2° 

+  9° 

88*° 

9° 

Anorthoclase 

1.540 

0.008 

+ 

77° 

o>p 

+  4° 

+  19i° 

74° 

19i° 

-15° 

-16° 

Albite 

1.542 

+ 

84° 

1)>P 

+  2i° 

+  11*° 

84i° 

13° 

-6° 

00 

Albite- 
oligoclase 

1.545 

- 

88°. 

p>u 

+  2° 

+  8° 

83° 

3° 

0° 

+  5° 

Oligoclase 

1.552 

1.557 
1.563 

1.568 

- 

86° 

p>u 

0° 

+  *° 

75° 

5° 

+  4° 

+  10° 

Oligoclase- 
andesine 

+ 

88° 

1)>P 

-2*° 

-8° 

68° 

7° 

+  13° 

+  19° 

Andesine 

+ 

77° 

1)>P 

—  5° 

-18° 

65° 

22° 

+  27° 

+  28° 

Labradorite 

+ 

77° 

u>p 

-9° 

-25° 

58° 

32° 

+  33° 

+  38° 

Labradorite- 
bytownite 

1.574 

0.010 

- 

80° 

p>u 

-21° 

-33° 

57° 

42° 

+  37° 

+  45° 

Bytownite 

1.588 

0.013 

- 

77° 

p>u 

-37° 

-36° 

55° 

48° 

+  43° 

+  56° 

Anorthite 

376 


PETROGRAPHIC  METHODS 


BIAXIAL 


Crys- 

Page 

2 

o 

Minerals 

Chemical 
composition 

tal 

sys- 

Cleav- 
age 

Develop- 
ment 

Sp. 

H 

Color 

a 

0 

tem 

| 

Phillipsite 

(CaK2)Al2 
Si5Oi4  +  5  aq. 

2.2 

~ 

M 

(001) 
(010) 

Spherulitic 

4.5 

Colorless 

I 

Harmo- 
tome 

(BaK2)Al2 
Si5Oi4  +  5  aq. 

2.5 

1.503 

1.506 

Laumont- 
ite 

CaAl2Si4Oi2 
+  4aq. 

M 

010 
110 

Prismatic 

2.4 

3.5 

Colorless 

1.513 

1.524 

Thomson- 
ite 

(CaNa2)Al2 
Si2Os  +  2J  aq. 

0 

010 
(100) 

Tabular  || 
{100} 

2.3 

5 

Colorless 

1.497 

1.503 

341 

a 

Heuland- 
ite 

CaAl2Si6Oi6 
+  5aq. 

Platy  I) 
{100} 

2.2 

5 

1.498 

1.499 

1 

Columnar 

Desmine 

(CaNa2)Al2 
Si6Oi6  +  6  aq. 

M 

010 

II  c 
Tabular  || 

2.15 

3.5 

Colorless 

1.494 

1.498 

1 

{010} 

Epistilbite 

CaAl2Si6Oi6 
+  5  aq. 

Radial 
Fibrous 

2.25 

4 

1.502 

1.510 

Natrolite 

Na2Al2SisOio 

2.2 

1.478 

1.481 

1 

O 

Mesolite 

M 

110 

Columnar 

II  « 

5.5 

Colorless 

About 

1 

A 

(89°) 

Radial 

1  .5 

1 

Scolesite 

CaAhSisOio 
+  3  aq. 

2.4 

1.502 

DESCRIPTIVEISECTION 


ZEOLITES. 


377 
TABLE  17 


r 

f-OL 

Chm 

Cbz 

2V 

2E 

Disper- 
sion 

Optical 
orientation 

Solu- 
bility 

Remarks 

Minerals 

0.003 

+ 

+ 

70° 

c:C  =  20° 
b-fi 

Readily 

Phillipsite 

Gelatin- 

o 

Very 

2 

weak 

TTPl 

§ 

1.508 

0.005 

+ 

85° 

c:0  =  60° 

b=B 

Difficult- 

Harmo- 
tome 

1.525 

0.012 

- 

- 

55° 

u^p 

c:0  =  70°f 

b=B 

Gelatinizes 
with 
HC1 

Decomposes 
in  air  with 
loss  of  water 

Laumont- 
ite 

1.525 

0.028 

* 

About 
90° 

u^p 

c=0 

b=  C 

Gelatinizes 
with 
HC1 

Fuses  with 
intumes- 
cence 

Thomson- 
ite 

1.505 

0.007 

- 

- 

0° 
to 
50° 

Crossed 

c    nearly  =  B 
b=C 

Gelatinizes 
with 
HC1 

Parallel 
aggregates 

Heuland- 
ite 

1.500 

0.006 

- 

- 

33° 

52° 

u>p 

c:0  =  8°  f 

Give 
powdered 

Often  radial 
aggregates 

Desmine 

silica 

1.512 

0.010 

- 

- 

40° 
to  70° 

u>p 

c:C  =  9°  r 

with  HC1 

Twinned 
||    {100} 

Epistil- 
bite 

1.490 

0.012 

- 

+ 

60° 

96° 

u>p 

c:C  =  0°-5° 

Radial 
aggregates 

Natrolite 

About 
0.01 

Gelatinize 
with 
HC1 

Twinning 

Mesolite 

lamellse 

0.008 

- 

- 

58° 

">' 

c:0  =  17°f 
b  =  C 

||    {100} 

Scolesite 

378  PETROGRAPHIC  METHODS 


TABLE  18 


ROCK  MINERALS  GROUPED  ACCORDING  TO  COLOR. 

Grayish-violet :  Perovskite,  tourmaline,  rutile,  titanium  augite,  cataphor- 
ite,  axinite. 

Violet :  Dumortierite,  piemontite,  cotschubeyite,  csemmererite,  fluorite. 

Blue  :  Riebeckite,  arfvedsonite,  carinthine,  dumortierite,  chlorite  (erinite), 
lazurite,  tourmaline,  glaucophane,  haiiyne-noselite,  lazulite,  chloritoid, 
serendibite,  crocidolite,  corundum,  fuchsite,  cordierite,  anatase,  cyanite, 
sapphirine,  sodalite,  beryl,  diaspore. 

Green:  Green  hornblende,  aegirine,  tourmaline,  glauconite,  seladonite, 
biotite,  aegirine-augite,  fassaite,  chrome  epidote,  iron  spinel,  pargasite,  brittle 
mica,  chlorite,  lime  garnet,  actinolite,  diopside,  baddeleyite,  chrysoberyl, 
serpentine. 

Orange  and  Yellow :  Brookite,  wurtzite,  sphalerite,  astrophyllite,  biotite, 
brown  hornblende,  rutile,  cassiterite,  lavenite,  humite  group,  staurolite, 
chrome  epidote,  rosenbuschite,  monazite,  glass,  carpholite,  baddeleyite, 
fayalite,  nontronite,  sulphur,  epidote,  griinerite,  cummingtonite,  tourma- 
line, apatite,  melilite,  lime  garnet,  anatase,  titanite,  rinkite,  mosandrite, 
prismatine,  siderite. 

Red:  Hematite,  piemontite,  chrome  chlorite,  thulite,  hypersthene,  cas- 
siterite, cataphorite,  titanite,  andalusite,  garnet,  orthite,  clinozoisite,  man- 
ganvesuvianite,  eudialyte,  xenotime. 

Brown :  Lievrite,  cossyrite,  ilmenite,  chromite,  pseudobrookite,  brookite, 
basaltic  hornblende,  biotite,  lithionite,  tourmaline,  rutile,  melanite,  orthite, 
chrome  spinel,  perovskite,  wurtzite,  sphalerite,  barkievikite,  hyalosiderite, 
astrophyUite,  fayalite,  staurolite,  brown  hornblende,  glass,  baddeleyite, 
hypersthene,  lime  garnet,  monazite,  diallage,  titanite,  pargasite,  antho- 
phyllite,  augite,  brittle  mica. 

N.  B.  In  a  general  way  the  minerals  are  arranged  above  according  to 
diminishing  intensity  of  color.  Those  at  the  beginning  of  a  series  are  in 
some  instances  not  very  transparent  while  those  at  the  end  scarcely  show  any 
color  at  all  in  thin  section.  Rare  varieties  are  introduced  here  when  the 
color  is  characteristic. 


00 


o    w 

P5     ^ 

3    PQ 


Double 

Refraction 

0.05  mm. 


0.04  mm. 


0.03  mm. 


0.02  mm. 


0.01  mm 


Retardation  200  400 

First  Order 


600  800  1000          | 

Second  Order 


Talc,  Fay  all  te 
Astrophyllite 
Epidote  f 
Biotite 
Anatase,  Zircon 


Basaltic  Hornblende 

Titanite  | 
Xenotime 
Cassiterite 

Titanite  f 

Aragonite,  Brookite 
Calcite,  Dolomite 
Magnesite 
Siderite 

0,287  Rutile 
0,290  Sulphur 


2000 
Fourth  Order 


2200 


2400  11. 


DESCRIPTIVE  SECTION  379 


TABLE  18.— Continued. 

I.  INTERFERENCE  FIGURES  ARE  OBSERVED  IN  CLEAVAGE 

PLATES. 

1.  Uniaxial  and  Nearly  Uniaxial: 

a.  Perpendicular  to  the  optic  axis:  Brittle  mica  ^p,  eudialyte  +,  alunite 
+  ,  phlogopite  — ,  biotite  — ,  pennine  ±,  brucite  +,  talc  — ,  apophyllite  +, 
heulandite  +. 

b.  Oblique  to  the  optic  axis:  Calcite,   dolomite,   magnesite,  siderite,  hy- 
drargillite. 

c.  Parallel  to  the  optic  axis:  Rutile,  wurtzite,  zircon,  xenotime,  scapolite, 
cancrinite. 

2.  Biaxial: 

a.  Perpendicular  to  the  acute  bisectrix,   approximate  optic  angle  in  paren- 
thesis: Brittle  mica  i  (not  very  large),  margarite  —  (large),  prehnite  + 
(quite  variable),  topaz  +   (quite  large,  variable),  celestite  +   (about  90°), 
muscovite  —  (about  70°),  sericite  —  (about  25°),  phlogopite  and  biotite  — 
(very  small),  lithionite  —  (variable),  clinochlore  +  (variable),  antigorite  — 
(variable),  talc  —   (about  25°),  pyrophyllite  —  ( >  100°),  hydromagnesite 
+    (120°),  heulandite  +  (small),   thomsonite   +  (about   90°),  goethite  + 
(crossed  optic  planes). 

b.  Perpendicular  to  the  obtuse  bisectrix:  Zoisite  /?  — ,   astrophyllite    — , 
anthophyllite  -,  pectolite  — ,  barite  -,  anhydrite  -. 

c.  Parallel  to  the  optic  plane:  Zoisite  a,  diaspore,  lawsonite,  sillimanite, 
humite,  bertrandite,  gypsum,  laumontite,  desmine,  epistilbite. 

d.  Slightly  inclined  to  the  acute  bisectrix  but  perpendicular  to  the  optic 
plane:  Monazite   +    (25°),  baddeleyite   —    (very  large),  rinkite   +    (77°), 
cyanite  —  (very  large),  clinochlore  +  (variable). 

e.  Greatly  inclined  to  the  acute  bisectrix  and  perpendicular  to  the  optic  plane: 
Hydrargillite,  clinozoisite. 

f.  Nearly  perpendicular  to  an  axis:  Epidote,  diallage,  wollastonite. 

g.  Inclined  to  the  optic  plane:  Brittle  mica,  amphibole,  augite,  wavellite, 
kaolin. 

II.  TWINNING  LAMINATIONS. 

(Those  indicated  with  *  do  not  often  show  lamination  on  account  of  the 
small  extinction  angle).  Leucite,  rutile,  perovskite,  calcite,  dolomite, 
baddeleyite,  zoisite,  *clinozoisite,  *epidote,  orthite,  cyanite,  chloritoid, 
margarite,  serendibite,  diallage,  rinkite,  mosandrite,  prehnite,  wollaston- 
ite, pectolite,  humite  group,  *mica,  clinochlore,  hydrargillite,  gypsum, 
hydromagnesite,  microcline,  plagioclase,  epistilbite,  mesolite,  scolesite. 


380  PETROGRAPHIC  METHODS 


TABLE  18. — Continued. 

HI.  MINERALS  THAT  FREQUENTLY  SHOW  ANOMALOUS 
INTERFERENCE  COLOR. 

Chrysoberyl,  clinozoisite,  zoisite  a,  pennine,  gehlenite,  melilite,  vesuvian- 
ite,  brucite,  dumortierite,  prehnite,  lime  garnet,  chloritoid,  f  assaite,  titanium 
augite,  laumontite,  sanidine. 

IV.  PLEOCHROIC  HALOS. 

Found  in :  Amphibole,  andalusite,  chlorite,  mica,  cordierite,  brittle  mica, 
staurolite,  tourmaline. 

Found  around:  Dumortierite,  orthite,  rutile,  titanite,  xenotime,  cassi- 
terite,  zircon. 

V.  USUAL  CRYSTALLOGRAPHIC  HABIT  OF  NON-CUBIC  MINERALS. 

1.  Isometric    (pyramidal,    thick    tabular,    short    prismatic,    granular): 
Anatase,    cassiterite,   xenotime,   gehlenite,    carbonates,   scapolite,    alunite, 
chabazite,    quartz,    nepheline,    titanite,    sulphur,    monazite,    chrysoberyl, 
diaspore,  forsterite,  olivine,  fayalite,  monticellite,  axinite,  rinkite,  lazulite, 
barite,  humite  group,  celestite,  topaz,  anhydrite,  cordierite. 

2.  Tabular :   Anatase,  corundum,  melilite,  brucite,  tridymite,  apophyllite; 
brookite,  pseudobrookite,  serendibite,  astrophyllite,  cyanite,  brittle  mica, 
margarite,    sapphirine,    spodumene,    axinite,    mosandrite,    micas,    chlorite 
group,  antigorite,  bertrandite,  hydrargillite,  talc,  kaolin,  heulandite. 

3.  Prismatic :    Rutile,  cassiterite,  zircon,  xenotime,  corundum,  vesuvian- 
ite,  tourmaline,  apatite,  scapolite,  beryl,  baddeleyite,  lievrite,  titanite,  epi- 
dote  group,  lavenite,  prismatine,  staurolite,  cyanite,  pyroxene  group,  am- 
phibole  group,  andalusite,  aragonite,  wollastonite  group,  wagnerite. 

4.  Columnar  (acicular) :    Tourmaline,  apatite,  cancrinite,  hydronephelite, 
gcethite,  lievrite,  acmite,  tremolite,  actinolite,  sillimanite,  datolite,  dumor- 
tierite,   aragonite,    wollastonite   group,    topaz,    celestite,    hydromagnesite, 
desmine,  natrolite. 

5.  Fibrous:    Wurtzite,    chalcedony,    sillimanite,    crocidolite,     prehnite, 
carpholite,  datolite,  chrysotile,  wavellite,  hydrargillite,  pyrophyllite,  gypsum, 
zeolites. 

VI.  CLASSIFICATION  OF  A  FEW  MINERALS  ACCORDING  TO 
DECREASING  MAGNETISM.     (AFTER  DOELTER.) 

Metallic  iron,  magnetite,  pyrrhotite;  ilmenite,  lievrite,  hematite;  chro- 
mite,  siderite,  almandine,  limonite,  augite  rich  in  iron,  iron  spinel,  arf  ved- 
sonite;  hornblende,  augite  poor  in  iron,  epidote,  pyrope;  tourmaline,  bronz- 
ite,  vesuvianite;  staurolite,  actinolite,  olivine,  pyrite;  biotite,  chlorite, 
rutile;  haiiyne,  diopside,  muscovite,  nepheline,  leucite,  dolomite,  feldspars. 


DESCRIPTIVE  SECTION  381 

TABLE  IS.— Continued: 

VII.  CLASSIFICATION  OF  ROCK-FORMING  MINERALS  ACCORDING 

TO  SOLUBILITY. 

1.  Insoluble    in    hydrofluoric    acid:     Magnetite,     chromite,    hematite, 
ilmenite,  graphite,  carbonaceous  substance,  perovskite,  spinel  group,  pris- 
matine,  astrophyllite,  fluorite,  rutile,  anatase,  cassiterite,  zircon,  xenotime, 
chrysoberyl,  corundum,  baddeleyite,  tourmaline,  serendibite,  beryl,  brookite, 
staurolite,  diaspore,  cyanite,  chloritoid,  xanthophyllite,  sapphirine,  axinite, 
sillimanite,  dumortierite,  andalusite,  topaz,  bertrandite. 

2.  Attacked  by  hydrochloric  acid  with  difficulty:     Hematite,  ilmenite, 
sphalerite,  boracite,  garnet,  vesuvianite,  pseudobrookite,  goethite,  titanite, 
orthite,  ottrelite,  lavenite,  carpholite,  lazulite,  hydrargillite,  labradorite. 

3.  With  hydrochloric  acid  give  powdered  silica:     Leucite,    meionite, 
apophyllite,  rinkite,  mosandrite,  anorthite,  desmine,  epistilbite. 

4.  With  hydrochloric  acid  give  gelatinous  silica :  Analcite,  sodalite  group, 
gehlenite,    melilite,    eudialyte,    nepheline,    hydronephelite,    lievrite,    for- 
sterite,  olivine,  fayalite,  monticellite,  datolite,  rosenbuschite,  wollastonite, 
pectolite,  humite  group,  chlorite  group,  serpentine,  heulandite,  natrolite, 
chabazite,  phillipsite,  harmotome,  laumontite,  thomsonite,  mesolite,  scolesite. 

5.  Soluble  in  hydrochloric  acid:    Pyrrhotite,  wurtzite,  magnetite,  peri- 
clase,  apatite,  *calcite,  *dolomite,  *magnesite,  *siderite,  brucite,  *cancrinite, 
monazite  (white  residue),  wagnerite,  wavellite,  *aragonite,  *hydromagnesite. 

Those  marked  *  evolve  carbonic  acid  with  effervescence. 

6.  Soluble  in  caustic  potash:     Opal    (quartz),    chalcedony,    tridymite, 
sulphur,  kaolin  (white  residue) . 

VIII.  CLASSIFICATION  OF  THE  MINERALS  ACCORDING  TO  THEIR 

FUSIBILITY  BEFORE  THE  BLOWPIPE. 

1.  Easily  fusible  (KobelPs  scale  1-3) :    Almandine,  grossularite,  lievrite, 
sulphur,    lavenite,     aegirine,     aegirine    augite,   spodumene,    glaucophane, 
riebeckite,  arfvedsonite,  crocidolite,    axinite,    rinkite,  datolite,  mosandrite, 
rosenbuschite,  pectolite,  lithionite,  phillipsite,  scolesite. 

2.  Fairly  easily  fusible  (3-4) :     Pyrope,  hessonite,  topazolite,  melanite, 
analcite,  tourmaline,  eudialyte,  scapolite,  elaeolite,  apophyllite,  chabazite, 
cancrinite,  hydronephelite,   epidote  group,   astrophyllite,   augite,  fassaite, 
lawsonite,  tremolite,  actinolite,  hornblende,  prehnite,  celestite,  lepidome- 
lane,     anhydrite,    nontronite,    gypsum,    anorthoclase,     basic    plagioclase, 
heulandite,  desmine,  natrolite,  thomsonite,  epistilbite. 

3.  Difficultly  fusible  (4-5)  :     Boracite,  sodalite,  melilite,  nepheline,  mona- 
zite, chloritoid,  fayalite,  hypersthene,  diopside,  diallage,  carpholite,  wollas- 
tonite,   muscovite,   phlogopite,  biotite,  clinochlore,  orthoclase,  microcline, 
acid  plagioclase,  harmotome. 


382  PETROGRAPHIC  METHODS 

TABLE  IS.— Continued. 

4.  Only  fusible  on  the  edges   (5-6):     Magnetite,   sphalerite,    wurtzite, 
haiiyne,  noselite,  fluorite,  gehlenite,  apatite,  beryl,  pseudobrookite,  baddel- 
eyite,    titanite,    margarite,    monticellite,    hyalosiderite,    enstatite,    antho- 
phyllite,  barite,  humite  group,  pennine,  serpentine,  cordierite,  wagnerite. 

5.  Infusible :    Perovskite,  spinel,  periclase,  leucite,  opal,  rutile,  anatase, 
cassiterite,  zircon,  xenotime,  corundum,  carbonates,  ahmite,  brucite,  quartz, 
chalcedony,   tridymite,    brookite,    chrysoberyl,  goethite,    serendibite,  pris- 
matine,  staurolite,  diaspore,  cyanite,  xanthophyllite,  sapphirine,  forsterite, 
olivine,    sillimanite,    dumortierite,    lazulite,    andalusite,    aragonite,    topaz, 
bertrandite,     hydrargillite,     talc,     pyrophyllite,     kaolin,     hydromagnesite, 
wavellite. 

IX.  FINE  POWDER  GIVES  ALKALINE  REACTION. 

Albite,  analcite,  andesine,  anorthite,  apophyllite,  axinite,  biotite,  bytown- 
ite,  datolite,  desmine,  epidote,  heulandite,  hydronephelite,  lime  garnet, 
clinochlore,  clinozoisite,  labradorite,  leucite,  lithionite,  margarite,  micro- 
cline,  muscovite,  natrolite,  nepheline,  oligoclase,  orthoclase,  pennine,  prehn- 
ite,  serpentine,  talc,  tremolite,  vesuvianite,  wollastonite,  zoisite. 


DESCRIPTIVE  SECTION 


383 


TABLE  19. 


X.   ROCK-FORMING 


MINERALS 
SPECIFIC 

Metallic  iron 7.8 

Galena 7.5 

Cassiterite 6.9 

Baddeleyite 5.8 

Hematite 5.25 

Magnetite 5.2 

Ilmenite 5.2-4.8 

Pyrite 5.0 

Pseudobrookite 5.0 

Zircon 4.7 

Pyrrhotite 4.6 

Xenotime 4.6 

Chromite 4.5 

Koppite 4.5 

Barite .  4.5 

Melanite 4.3 

Almandine 4.3 

Pyrochlore 4.3 

Goethite 4.3 

Rutile 4.25 

Chalcopyrite 4.2 

Chrome  spinel 4.1 

Topazolite 4.1 

Dysanalyte 4.1 

Brookite.. 4.1-3.9 

Sphalerite 4.0 

Wurtzite 4.0 

Lievrite 4.0 

Perovskite 4.0 

Corundum 4.0 

Celestite 3.95 

Iron  spinel 3.9 

Siderite 3.9 

Anatase 3.9 

Hessonite 3.8 

Staurolite 3 . 8-3 . 6 

Pyrope 3.75 

Chrysoberyl 3.7 

Periclase 3.65 

Cyanite 3.6 

Ardennite 3.6 

Orthite 3.6 

Spinel 3.6 

Lavenite 3.6 

Fayalite 3.6 

Topaz 3.6-3.4 

Rohrbach  solution 3-58 

Diamond. .  .3.5 


ARRANGED    ACCORDING     TO 
GRAVITY. 

Chloritoid 3.5 

Titanite 3.5 

Sapphirine 3.5 

Aegirine 3.5 

Arf vedsonite 3.5 

Triphyline 3.5 

Hypersthene 3.5 

Diaspore 3 . 45 

Rinkite 3.45 

Grossularite 3 . 45 

Vesuvianite 3 . 45 

Woehlerite 3.4 

Barkievikite 3.4 

Serendibite 3.4 

Epidote 3.4 

Augite 3.35 

Astrophyllite 3 . 35 

Prismatine 3 . 35 

Jadeite 3.35 

Clinozoisite 3 . 35 

Methylene  iodide 3.32 

Zoisite 3.3 

Diallage 3.3 

Olivine 3.3 

Bronzite 3.3 

Basaltic  hornblende 3.3 

Axinite.. 3.3 

Riebeckite 3.3 

Dumortierite 3.3 

Rosenbuschite 3.3 

Diopside 3.3 

Klein's  solution 3.28 

Cornerupine 3 . 25 

Sillimanite 3 . 25 

Forsterite 3 . 25 

Tourmaline 3.25-3.0 

Crocidolite 3.2 

Fluorite.... 3.2 

Andalusite 3.2 

Monticellite 3.2 

Fuggerite. 3.2 

Common  hornblende 3 . 2-3 . 1 

Humite 3.2-3.1 

Chondrodite 3 . 2-3 . 1 

Clinohumite 3 . 2-3 . 1 

Fassaite 3.2-3.0 

Biotite. 3 . 2-3 . 0 

Lithionite..  ..3.2-2.8 


384 


PETROGRAPHIC  METHODS 


TABLE  19. —Continued. 

X.     ROCK-FORMING     MINERALS    ARRANGED    ACCORDING 
SPECIFIC  GRAVITY.— Continued. 


TO 


Thoulet  solution 3.18 

Apatite 3.15 

Enstatite 3 

Lazulite 3 

Magnesite 3 

Spodumene 3 

Lawsonite 3 

Euclase 3 

Xanthophyllite 3 

Anthophyllite 3 

Glaucophane 3 

Mosandrite 3 

Eucolite 3.05 

Actinolite 3.0 

Phlogopite 3.0 

Wagnerite 3.0 

Datolite 3.0 

Boracite 3.0 

Cryolite 3.0 

Phenacite 3.0 

Margarite 3.0 

Acetylene  bromide 3.0 

Dolomite 2 . 95 

Carpholite 2.95 

Aragonite 2 . 95 

Anhydrite 2.95 

Melilite 2.9 

Prehnite 2.9 

Pectolite 2.9 

Tremolite 2.9 

Chlorite .2.9-2.6 

Conchite 2 . 85 

Wollastonite 2.85 

Paragonite 2 . 85 

Muscovite 2 . 85 

Iddingsite 2.85 

Pyrophyllite 2.8 

Anorthite 2 . 75 

Scapolite 2.75-2.6 

Bytownite 2 . 72 

Calcite 2.72 

Alunite 2.7 

Beryl 2.7 

Talc 2.7 

Pennine 2.7 

Clinochlore 2.7 

Nontronite. .  .  2.7 


Serpentine 2 . 7-2 . 5 

Labradorite 2 . 69 

Andesine 2 . 67 

Quartz 2.65 

Cordierite 2 . 65 

Ktypeite 2.65 

Glass.  . 2.65-2.2 

Oligoclase 2 . 64 

Albite 2.60 

Chalcedony 2.6 

Bertrandite 2.6 

Nepheline 2.6 

Anorthoclase 2.58 

Orthoclase 2 . 56 

Microcline 2 . 56 

Leucite. .  .  2.5 


Haiiyne. 


2.5 


Kaolin 2.5 

Harmotome 2.5 

Cancrinite 2.45 

Hydrargillite 2.4 

Wavellite 2.4 

Brucite 2.4 

Laumontite 2.4 

Apophyllite 2.35 

Sodalite 2.3 

Graphite 2.3 

Tridymite 2.3 

Gypsum 2.3 

Glauconite 2.3 

Scolesite 2.3 

Thomsonite 2.3 

Epistilbite 2.25 

Analcite 2.2 

Hydronephelite 2.2 

Natrolite 2.2 

Opal 2.2 

Heulandite 2.2 

Phillipsite 2.2 

Desmine 2.15 

Hydromagnesite 2.15 

Chabazite 2.1 

Halite 2.1 

Gmelmite 2.1 

Sulphur 2.0 

Meerschaum 2.0 

Coals..  ....<2.0 


INDEX 


a-dimethyl  glyoxime,  167 

Abbe,  drawing  apparatus,  134 

test  plate,  24 

total  reflect ometer,  39 
Abnormal  interference  colors,  78 
Accessory  apparatus,  125 
Achromatic,  3 
Acmite,  285 
Actinolite,  289 
Acute  bisectrix,  59 
Adjustment  of  the  cross  hairs,  28 
Adjustment  of  the  nicols,  28 
Adularia,  328 
Aegirine,  284 
Aegirine  augite,  284 
Aenigmatite,  294 
Agalmatolite,  317 
Aggregate,  185 
Aggregate  polarization,  186 
Alaunschiefer,  205 
Albite,  330 
Albite  law,  332 
Alidade,  40 
Alkali  feldspar,  324 
Allanite,  261 
Allotriomorphic,  181 
Almandite,  215 
Alum,  separation  by,  151 
Aluminium,  chemical  test  for,  168 
Aluminium  fluosilicate,  164 
Alunite,  243 
Amazon  stone,  329 
Amesite,  312 

Ammonium,  chemical  test  for,  165 
Ammonium  metavanadinate,  170 
Ammonium  phosphomolybdate,  171 
Amphibole,  alteration  of,  292 

color  of,  in  a  slide,  293 

extinction  of,  293 

group,  287 


Analcite,  222 

Analyses  by  washing,  152 

Analyzer,  13,  51 

Anastigmatic  lens,  3 

Anatase,  229 

Andalusite,  297 

Andesine,  335 

Angle  of  aperture  of  objectives,  98 

Anhydrite,  301 

Anigmatite,  256 

Anisotropic,  52 

Anomite,  307 

Anorthite,  330 

Anorthoclase,  329 

Anthophyllite,  289 

Antigorite,  276,  312 

Antiperthite,  326 

Apatite,  236 

Apertometer,  Abbe,  25 

Aperture,  6 

Aperture  and  magnification,  25 

Aphanitic,  183 

Aplanatic,  3 

Aplanatic  and  achromatic,  test  for, 
24 

Apochromatic,  5 

Apophyllite,  250 

Apparatus  for  separating  rock  pow- 
der, 155 

Apparent  optic  angle,  112 

Aragonite,  299 

Arfvedsonite,  293 

Astrolite,  309 

Astrophyllite,  269 

Attractive  crystals,  57 

Augite,  282 

Authigenetic,  228 

Automorphic,  181 

Awaruite,  206 

Axinite,  295 

Axiolites,  186 


385 


386 


INDEX 


Babinet  compensator,  91 

Baculite,  44,  189 

Baddeleyite,  255 

Barite,  297 

Barium,  chemical  test  for,  166 

Barium  fluosilicate,  164 

Barium  oxalate,  166 

Barkevikite,  293 

Basaltic  augite,  282 

Basaltic  hornblende,  287,  291 

Bastite,  279,  314 

Baveno  law,  326 

Becke's  method,  33 

Beckelite,  213 

Belonite,  183 

Bertrand  lens,  98,  113 

Bertrand  plate,  72 

Bertrandite,  317 

Beryl,  243 

Beryllium,  chemical  test  for,  166 

Biaxial  crystals,  58 

negative,  59 

positive,  59 
Biaxial  interference  figure,  97,  107, 

et  sequa 

Biaxial  zeolites,  341 
Binocular  microscope,  41 
Biot,  quartz  plate,  73 

rotating  quartz,  92 
Biotite,  302 
Birefractometer,  91 
Bisectrices,  59 
Bituminous  matter,  212 
Blue  spar,  298 
Bohemian  garnet,  216 
Bone  substance,  237 
Boracite,  219 

Boron,  chemical  test  for,  169 
Botryolite,  296 
Bowlingite,  315 
Brandisite,  271 
Bravais  double  plate,  72 
Breislakite,  256 
Brewster's  cross,  123 
Brewster  lens,  2 
Brezina  double  plate,  71 
Brilliant  green,  175 
Brittle  mica  group,  269 
Bronzite,  279 


Brookite,  254 
Brown  hornblende,  291 
Brucite,  244 
Bruecke  lens,  3 
Bytownite,  335 

Caesium,  chemical  test  for,  165 

Caesium  alum,  169 

Calcination,  176 

Calcite,  238 

Calcium,  chemical  test  for,  165 

Calcium  fluosilicate,  164 

Caldron  double  plate,  71 

Cammererite,  310 

Canada  balsam,  143 

Cancrinite,  250 

Carbon,  chemical  test  for,  169 

Carbonaceous  matter,  210 

distinction  from  graphite,  211 

Carinthine,  290 

Carlsbad  law,  326 

Carpholite,  299 

Carrara,  239 

Cassiterite,  230 

Cataclase,  192 

Cataphorite,  288,  293 

Cedar  oil,  143 

Celestite,  299 

Centering  screws,  14 

Centering  the  stage,  26 

Cerium,  chemical  test  for,  168 

Cerium  epidote,  261 

Ceylon,  210 

Chabazite,  250 

Chalcedony,  248 

Chalcopyiite,  205 

Chamosite,  312 

Character  of  the  double  refraction 
in  biaxial  minerals,  115 
in  uniaxial  minerals,  105 

Character    of    the    principal    zone, 
determination  of,  93 

Characteristic  angles,  measurement 
of,  42 

Characteristic  colors,  49 

Chatoyancy,  196 

Chemical  methods  of  investigation, 
162 

Chemical  microscope,  130 


INDEX 


387 


Chemical  reactions,  aluminium  fluo- 
silicate,  164 

barium  fluosilicate,  164 

calcium  fluosilicate,  164 

fluosilicic  acid,  163 

lithium  fluosilicate,  164 

magnesium  fluosilicate,  164 

potassium  fluosilicate,  163 

sodium  fluosilicate,  163 

strontium  fluosilicate,  164 
Chemical  tests  for  aluminium,  168 

ammonium,  165 

barium,  166 

beryllium,  166 

boron,  169 

caesium,  165 

calcium,  165 

carbon,  169 

cerium,  168 

chlorine,  171 

chromium,  168 

cobalt,  167 

didymium,  168 

erbium,  168 

fluorine,  171 

iron,  167 

lanthanum,  168 

lithium,  165 

magnesium,  166 

manganese,  168 

molybdenum,  168 

neodymium,  168 

nickel,  167 

niobium,  170 

phosphorus,  171 

potassium,  165 

praseodymium,  168 

rubidium,  165 

silicon,  169 

sodium,  164 

strontium,  166 

sulphur,  171 

tantalum,  170 

thorium,  168 

tin,  170 

titanium,  170 

tungsten,  168 

vanadium,  170 

water,  172 


Chemical  tests  for,  yttrium,  168 

zirconium,  170 
Chiastolite,  297 

Chlorine,  chemical  test  for,  171 
Chlorite  group,  310 
Chloritoid,  271 
Chloropal,  319 

Chlorous  acid,  separation  by,  150 
Chondrodite,  300 
Chromatic  aberration,  2 
Chrome,  diopside,  280 

epidote,  261 

mica,  309 

ocher,  207,  309 

spinel,  218 

zoisite,  261 
Chromite,  207 

Chromium,  chemical  test  for,  168 
Chromoscope  for  interference  colors, 

84 

Chrysoberyl,  257 
Chrysotile,  275,  312 
Cinnamon  stone,  216 
Circular  polarization,  104 

strength  of,  104 
Cleavage,  179,  190 

determination  of,  41 

distinct,  44,  191 

fibrous,  191 

imperfect,  44,  191 

perfect,  44,  191 
Clinochlore,  310 
Clinohumite,  300 
Clinozoisite,  259 
Clint onite,  271 
Cobalt,  chemical  test  for,  167 
Coddington  lens,  2 
Color,  48 

Colors  of  the  axes,  61 
Common,  garnet,  215 

hornblende,  287 

spinel,  218 

Comparator,  quartz  wedge,  81 
Compensators,  88 

use  of,  93 
Compound  microscope,  diagram  of 

rays  through,  4 
Concentric  structure,  194 
Conchite,  241 


388 


INDEX 


Condenser,  7,  19 
Congo  red,  173 
Convergent  light,  95 

field  of  vision  in,  96 

methods  of  observation  in,  97 
Copiapite,  319 
Cordierite,  319 
Corundum,  232 
Cossaite,  306 
Cbssyrite,  294 
Cotschubeyite,  310 
Couseranite,  242 
Cover-glass,  22 
Crocidolite,  292 

Cross  sections  of  biaxial  minerals, 
252 

of  cubic  minerals,  212 

of  uniaxial  minerals,  225 
Crossed  dispersion,  112 
Crossed  nicols,  65 
Crossite,  294 
Cryptoperthite,  327 
Crystal  sandstone,  246 
Crystallites,  44,  189 
Crystallizing  microscope,  130 
Cummingtonite,  290 
Cumulite,  44,  189 

Cutting  and  polishing  machine,  145 
Cyanite,  267 

Damourite,  306 

Datolite,  296 

Delessite,  312 

Demantoid,  217 

Desmine,  341 

Determination  of  index  of  refraction 

by  raising  the  tube,  33 
Development  of  rock  constituents, 

181 

Diabase  augite,  282 
Diaclasite,  280 
Diallage,  281 
Diaspore,  267 
Diatoms,  26 
Dichroism,  62 
Dichroscopic  ocular,  61 
Didymium,  chemical  test  for,  168 
Dilute  colors,  49 
Diphyre,  242 


Dispersion,  60 

colors,  79,  110 

of  the  optic  axes,  109 

of  ordinary  light,  2 
Disthene,  267 
Dolomite,  238,  240 
Dolomite  ash,  241 
Double  refraction,  51 

in  calcite,  52 

recognition  of,  66 

strength  of,  73 

of  uniaxial  crystals,  56 
Drawing  apparatus,  Abbe,  134 

Nachet,  135 
Dry  system,  6 
Duelho,  317 

Duke  de  Chaulnes'  method,  46 
Dumortierite,  295 
Dysanalyte,  213 

Edenite,  290 

Eichstaedt  apparatus,  152 

Eighth  undulation  plate,  107 

Elaeolite,  249 

Elasticity,  axes  of,  58 

direction  of  greatest,  57 
direction  of  least,  57 
direction  of  medium,  58 

Electro-magnet,  160 

Ellipsoid  of  rotation,  57 

Emerald,  244 

Emery,  232 

Enstatite,  279 

Eozoon,  276 

Eozoon  canadensis,  240 

Epidosite,  264 

Epidote,  261 

Epidote  fels,  264 

Epidote  group,  258 

Epistilbite,  341 

Erbium,  chemical  test  for,  168 

Erinite,  310 

Eucolite,  242 

Eudialyte,  242 

Eugenol,  143 

Eutaxite,  222 

Eutectic  mixture,  186 

Exner  refractometer,  38 

Extinction,  curve  for  diopside,  70 


INDEX 


389 


Extinction,  directions,  67 

oblique,  69 

parallel,  69 

position  of,  67 

symmetrical,  69 
Extraordinary  ray,  52 

Fassaite,  281 
Fayalite,  272 
Fedorow,  mica  wedge,  90 

universal  stage,  127 
Fedorowite,  285 
Feldspar,  alteration  of,    323 

cleavage  of,  323 

development  of,  322 

dimensions  of,  322 

group,  321 
Felsite,  222 
Ferrite,  208 
Fibrolite,  295 

Fluorine,  chemical  test  for,  171 
Fluorite,  225 
Fluosilicic  acid,   chemical  reactions 

with,  163 
Foraminifera,  187 
Form,  determination  of,  41 
Forsterite,  272,  276 
Fouque  method  for  determining  the 

plagioclase,  338 
Fourlings,  120 
Fraunhofer  lens,  3 
Fresnel's  hypothesis,  54 
Fritting,  220 
Fuchsin,  175 
Fuchsite,  309 
Fuggerite,  234 

Galena,  205 
Garbenschiefer,  290 
Garnet  group,  214 
Gas  inclusions,  47 
Gastaldite,  292 
Gauss  mirror,  43 
Gedrite,  289 
Gehlenite  group,  233 
Genthite,  316 
Gieseckite,  250 
Gigantolite,  320 


Glass,  220 

Glass  inclusions,  48 

Glauconite,  309 

Glaucophane,  292 

Globulite,  44,  189 

Goethite,  254 

Goniometer,  reflection,  43 

Granophyre,  248,  327 

Granospherite,  187 

Graphite,  209 

Graphitic  acid,  210 

Graphitite,  209 

Graphitoid,  209 

Grapholite,  317 

Green  earth,  309 

Green  hornblende,  289 

Green  sand,  309 

Greisen,  225 

Grenough     binocular      microscope, 

41 

Grossularite,  217 
Grothite,  256 
Grunerite,  290 
Giimbelite,  317 
Gypsum,  321 

Haidinger  lens,  61 

Hainite,  257 

Half  shadow  polarizer,  73 

Hardness,  determination  of,  179 

Harmotome,  341 

Haiiyne,  223 

special  test  for,  174 
Heating  apparatus,  129 
Heavy  molten  liquids,  159 
Heavy  organic  liquids,  156 
Heavy  solutions,  157 
Heavy  spar,  297 
Hedenbergite,  280 
Helicoidal  structure,  196 
Hematite,  207 
Hepar  test,  171 
Hercynite,  218 
Hessonite,  217 
Heulandite,  341 
Hiddenite,  286 
Horizontal  dispersion,  111 
Hourglass  structure,  188 
Humboldite,  233 


390 


INDEX 


Humite,  300 
Humite  group,  300 
Hyalomelane,  221 
Hyalosiderite,  273,  274 
Hydatogenic  activity,  283 
Hydatopyrogenic,  194 
Hydrargillite,  316 
Hydrochloric   acid,    separation   by, 

150 
Hydrofluoric    acid,    separation  by, 

149 
Hydrofluosilicic  acid,  separation  by, 

150 

Hydromagnesite,  319 
Hydronephelite,  251 
Hypersthene,  279 
Hypidiomorphic  development,  181 

Iddingsite,  276,  315 
Idiomorphic  development,  181 
Illuminating  apparatus,  19 
Ilmenite,  208 
Ilvaite,  256 
Immersion  system,  6 
Inclined  dispersion,  111 
Inclusions,  47,  194 

carbon  dioxide,  194 

gas,  47 

glass,  48 

liquid,  48 

Index  of  refraction,   determination 
by  immersion  method,  36 

determination  by  the  method  of 
Schroeder  van  der  Kolk,  38 

determination  by  using  center 
screen,  39 

law  of,  30 

methods  of  determining,  30 
Indol,  212 
Interference  colors,  74 

abnormal,  78 

measurement     of    the     double 
refraction  by,  79 

modification  of,  77 

order  of,  76 

subnormal,  78 

supernormal,  78 

with  parallel  nicols,  74 

without  using  analyzer,  65 


Interference  figure,   biaxial,    107,   ct 
sequa 

of  low  double  refracting  min- 
erals, 103 

uniaxial,  100  • 

Interior  conical  refraction,  83 
Interstitial  material,  182 
Investigation,  chemical  methods  of, 
162 

physical  methods  of,  177 
lodocrase,  233 
Iron  cap,  321 
Iron,  206 

chemical  test  for,  167 
Iron  spinel,  218 
Isotropic  substances,  52 
Itacolumite,  246 
Ivaarite,  217 

Jadeite,  286 
Johnstrupite,  297 

Kaolin,  318 
Karsutite,  292 
Kelyphite,  216 
Klein's  lens,  113 
Klein's  solution,  158 
Knopite,  213 
Knotenschiefer,  320 
Kobell  stauroscope,  71 
Koppite,  213 
Kornerupine,  269 
Kreittonite,  218 
Ktypeite,  241 
Kuntzite,  286 

Labradorite,  335 

Lanthanum,  chemical  test  for,  168 

Lapis  lazuli,  224 

Lasaul  x  method,  97 

Lattice  lamination,  122,  185 

Lattice  structure,  in  microcline,  329 

in  olivine,  276 

of  serpentine,  313 
Laumonite,  341 
Lavenite,  257 
Lawsonite,  286 
Lazulite,  298 
Lazurite,  224 


INDEX 


391 


Lead  chloride,  159 

Leeson's  prism,  42 

Left  handed  crystals,  104 

Lens,  1 

Lens  stand,  161 

Lepidomelane,  307 

Leuchtenbergite,  243,  310 

Leucite,  219 

Leucite  basalt,  219 

Leucite  syenite,  219 

Leucite  tephrite,  219 

Leucitophyre,  219 

Leucoxene,  208,  228 

Lherzolite,  218 

Liebenerite,  250 

Lievrite,  256 

Lime  garnets,  217 

Limonite,  254 

Limurite,  295 

Linck's  solution,  175 

Liquid  inclusions,  48 

.Listwanite,  317 

Lithionite,  302 

Lithium,  chemical  test  for,  165 

Lithium  fluosilicate,  164 

Lithium  mica,  302 

Lotrite,  299 

Lussatite,  248 

Lutezite,  248 

Magmatic  corrosion,  189 
Magmatic  resorption,  288 
Magnesite,  241 

Magnesium,  chemical  test  for,  166 
Magnesium  ammonium  phosphate, 

167 

Magnesium  fluosilicate,  164 
Magnesium  mica,  302 
Magnetic  separation,  160 
Magnetite,  206 
Magnetites  and,  206 
Malachite  green,  173 
Malacolite,  281 
Malacon,  231 
Mallard  law,  113 

Manganese,  chemical  test  for,  168 
Margarite,  44,  189,  272 
Margarodite,  306 
Marialite,  242 


Masonite,  271 

Mastic,  146 

Measurement  of  double  refraction,  79 

Mechanical  stage,  18 

Meionite,  242 

Melanite,  217 

Melilite,  233 

Mercurous  nitrate,  159 

Meroxene,  307 

Mesh  structure  in  olivine,  275 

Mesolite,  341 

Mesostasis,  182 

Metaxite,  313 

Methods  of  investigation,  162 

Methods  of  separatiod,  148 

Methylene  iodine,  156 

Mica,  alteration  of,  308 

first  order,  303 

group,  302 

palme",  306 

second  order,  303 

test  plate,  90 

wedge,  90 
Microcline,  329 

perthite,  329 

Microfelsite,  222,  248,  327 
Microlites,  183 
Micrometer,  object,  45 

ocular,  45 

Micropegmatite,  247,  327 
Microperthite,  326 
Microphotographic  apparatus,  132 
Microscope,  electric  heating,  131 
Microscope  goniometer,  125 
Microtine,  334 
Microtome,  21 
Molybdenite,  206 

Molybdenum,  chemical  test  for,  168 
Monazite,  257 
Monticellite,  272,  276 
Mortar  structure,  191 
Mortel  structure,  191 
Mosaic  structure,  187 
Mosandrite,  297 
Muscovite,  302 
Myrmecitic  intergrowth,  248 
Myrmecoidal,  327 

Nachet  drawing  apparatus,  135 


392 


INDEX 


Natrolite,  341 
Needle  iron  ore,  254 
Negative  crystals,  57,  105,  195 
Neodymium,  chemical  test  for,  168 
Nepheline,  249 

special  test  for,  174 
Nephrite,  233,  286,  289 
Nickel,  chemical  test  for,  167 
Nicol,  11 

construction  of,  10 
Niobium,  chemical  test  for,  170 
Nontronite,  319 
Noselite,  224 

special  test  for,  174 
Numeite,  316 
Numerical  aperture,  6 
Nummulites,  187 
Nutrition  canals,  237 

Object,  clamp,  19 

glasses,  23 

marker,  19 

micrometer,  45 
Objective  holders,  5 
Obsidian,  220 
Obtuse  bisectrix,  59 
Ocellary  structure,  193 

in  leucite,  220 
Ocular,  diaphragm,  99 

goniometer,  43 

micrometer,  45 

Planimeter,  46 

Ramsden,  46 
Oldhamite,  213 
Oligoclase,  335 
Olivine,  272 
Olivine  group,  272 
Omphazite,  280 
Onion  structure,  221 
Onkosine,  306 
Onyx,  299 
Oolite,  186 
Opal,  224 

Opal  sandstone,  224 
Opaque  bodies,  49 
Opazite,  207 
Ophicalcite,  315 
Optic  angle,  59 

apparent,  112 


Optic  angle,  measurement  of,  112 

scale,  114 
Optic  axis,  55 
Optic  normal,  59 
Optical  anomalies,  123 
Optical  anomalies  in  garnet,  214 
Optical  sketch  of  a  crystal,  138 
Ordinary  light,  observations  in,  30 
Ordinary  ray,  52 
Organic  acids,  separation  by,  150 
Organic  liquids,  heavy,  156 
Orthite,  261 
Orthoclase,  324 
Orthorhombic  dispersion,  110 
Orthoscope,  Noerremberg,  9 
Ottrelite,  270 
Owenite,  312 

Palimsest  structure,  198 

Paragonite,  306 

Paragonite  schist,  268 

Parallel  polarized  light,  observations 

in,  51 

Paramorph,  123,  198 
Pargasite,  287,  290 
Parting,  191 
Pectolite,  300 
Peg  structure,  234 
Pennine,  310  . 
Periclase,  219 
Pericline  law,  333 
Peridote,  272 
Perimorph,  197 
Peripheral  development,  192 
Periscopic  ocular,  5 
Perlite,  220 
Perovskite,  213 
Perthite,  326 
Petrographic  methods,  summary  of, 

135 

Phillipsite,  341; 
Phlogopite,  302 
Phosphorite,  237 

Phosphorus,  chemical  test  for,  171 
Photographic  camera,  133 
Physical  methods  of  investigation, 

177 

Picotite,  218 
Picrolite,  313 


INDEX 


393 


Picrosmine,  313 
Piemontite,  261 
Pilite,  274,  289 
Pimelite,  316 
Pistazite,  261 
Pitchstone,  220 
Plagioclase,  330 
Plane  of  the  optic  axes,  59 
Plane  of  polarization,  7 
Plane  of  vibration  of  polarized  light,  8 
Plane  polarized  light,  7 
Planimeter  ocular,  46 
t      Pleochroic  halos,  64,  197 

in  hornblende,  288 
Pleochroism,  61 

artificial,  64 
Pleonast,  218 

Pleurosigma  angulatum,  26 
Poikilitic  intergrowth,  192 
Polariscope,  Noerremberg,  9 
Polarization,  angle  of,  9 

by  reflection,  9 

by  refraction,  9 
Polarizer,  19,  51 
Polarizing  apparatus,  7 
Polarizing  microscope,      Bausch     & 
Lomb,  18 

Nachet,  14 

polymeter,  17 

Rei  chert,  15 

Seibert,  12 

Voigt  &  Hochgesang,  16 
Porfido  rosso  antico,  261 
Positive  crystals,  57 
Potassium  antimonate,  169 
Potassium,  chemical  test  for,  165 
Potassium  ferrocyanide,  167 
Potassium  fluoborate,  169 
Potassium  fluosilicate,  163 
Potassium  mercuric  iodide,  37 
Potassium  mica,  302 
Potassium  platinochloride,  165 
Potassium  sulphocyanide,  167 
Potstone,  316 
Priigratite,  306 
Praseodymium,    chemical    test    for, 

168 

Precious  garnet,  215 
Precious  serpentine,  315 


Precious  spinel,  218 
Precipitates  on  thin  section,  175 
Predazzite,  219,  244 
Prehnite,  299 
Principal  rays,  180 
Principal  section,  53 
Principal  zone,  63 
Prismatine,  269 
Projection  apparatus,  133 
Protobastite,  280 
Protoclase,  191,  324 
Prussian  blue,  167 
Pseudobrookite,  255 
Pseudochalcedony,  249 
Pseudochroism,  49 
Pseudodichroism,  65 
Pseudomorphs,  198 
Pumice,  221 
Pycnite,  301 
Pyrenaite,  217 
Pyrite,  204 
Pyrochlore,  213 
Pyrope,  216 
Pyrophyllite,  317 
Pyroxene  group,  277 
Pyroxenes,  monoclinic,  280 

orthorhombic,  279 
Pyrrhite,  213 
Pyrrhotite,  205 

Quarter  undulation  plate,  90 
Quartz,  244 

au'gen,  245 

vermicule,  248 

wedge,  90 
Quartzine,  248 

Radde  color  scale,  49,  63 
Radio  active  minerals,  64 
Radiotine,  313 
Ramsden  ocular,  46,  99 
Ranite,  251 
Raven  mica,  308 
Real  image,  1 
Red  I.  test  plate,  90 
Red  II.  test  plate,  90 
Reflected  light,  observations  in,  49 
Reflection  in  the  tube  of  the  micro- 
scope, 26 


394 


INDEX 


Refraction,  51 

Refractometer,  Exner,  38 

Relation    of    optical    character    to 

character  of  the  principal 

zone,  94 

Repellant  crystals,  57 
Resolving  power  of  objectives,  6 
Rhaetizite,  267 
Rhodochrome,  310 
Rhomb ohedral  carbonates,  238 
Rhomb  porphyry,  322 
Riebeckite,  293 
Right  handed  crystals,  104 
Rinkite,  295 
Ripidolite,  312 

Rock  constituents,  size  of,  183 
Rock  grinding  machine,  21 
Rock  powder,  143 
Rock  salt,  219 
Rohrbach  solution,  159 
Rosenbuschite,  300 
Rotation  apparatus,  125 

C.  Klein,  128 
Rubellane,  307 

Rubidium,  chemical  test  for,  165 
Ruby,  232 
Ruby  mica,  254 
Ruin  development,  188 
Rutile,  227 

Sagenite,  228,  308 

Salite,  281 

Sanidine,  324,  328 

Saponlac,  146 

Sapphire,  232 

Sapphirine,  268 

Saussurite,  263,  336 

Saussuritization,  336 

Scale  for  specific  gravity,  154 

Scapolite  group,  242 

Schaffgotsch,  Count,  157 

Schorl,  234 

Schorlomite,  217 

Schroeder  van  der  Kolk  method  for 

determining     the     plagio- 

clase,  339 
Schungite,  211 
Schwarzmann's    optic    angle    scale, 

114 


Scolesite,  341 

Sedgy  hornblende,  290 

Seladonite,  284,  309 

Sesnitive  red  tint,  76 

Sensitive  tint  plate,  72 

Separating  funnel,  156 

Separation     according    to     specific 

gravity,  153 

Separation,    chemical    methods    of, 
148 

magnetic,  160 

physical  methods  of,  152 
Separation  of  rock  constituents  by 
alum,  151 

chlorous  acids,  150 

hydrochloric  acid,  150 

hydroflouric  acid,  149 

hydrofluosilicic  acid,  150 

organic  acid,  150 

sodium  hydroxide,  151 

sulphuric  acid,  151 
Serendibite,  269 
Sericite,  305 
Sericite  schist,  306 
Serpentine,  312 
Seyberite,  271 
Siderite,  241 
Sieve  structure,  197 
Silicon,  chemical  test  for,  169 
Silicon  tetrafluoride,  169 
Sillimanite,  295 
Silver  nitrate,  159 
Sismondine,  271 
Size,  measurement  of,  45 
Skarn,  217,  280 
Skatol,  212 
Skeletal  crystals,  44 
Slag  inclusions,  195 
Smaragdite,  290 
Soapstone,  316 
Soda  hornblende,  288 
Soda-lime  feldspar,  330 
Sodalite,  223 
Sodalite  group,  223 
Sodalite,  special  test  for,  174 
Soda  mica,  306 
Soda  orthoclase,  329 
Sodium,  chemical  test  for,  164 
Sodium  fluosilicate,  163 


INDEX 


395 


Sodium  hydroxide,    separation    by, 

151 

Sodium  saltpeter  nicols,  11 
Sodium  trithiocarbonate,  167 
Sodium  uranylacetate,  164 
Solenhofener  schist,  238 
Sonnenstein,  208 

Source  of  light  for  microscopic  ob- 
servations, 7 
Special  reactions,  172 
Special  tests  for  Hauyne,  174 

Nepheline,  174 

Noselite,  174 

Scapolite,  174 

Sodalite,  174 

Specific  gravity  scale,  154 
Spectro-polarizer,  Abbe,  7 
Spessartite,  216 
Sphalerite,  213 
Sphene,  256 
Spherical  aberration,  2 
Spherosiderite,  241 
Spherulites,  186 
Spinel  group,  218 
Spodumene,  286 
Spreustein,  223,  250,  251 
Staining  methods,  173 
Stanhope  lens,  2 
Staurolite,  266 
Stauroscopes,  71 
Steinheil  triplet,  3 
Stereoscopic  microscope,  41 
Stereoscopic  ocular,  41 
Stilbite  group,  341 
Stink  quartz,  246 
Strontium,  chemical  test  for,  166 
Strontium  fluosilicate,  164 
Structure,  concentric,  194 

dodecahedral  in  garnet,  215 

helicoidal,  196 

hourglass,  188 

mosaic,  187 

onion-like,  221 

palimsest,  198 

peg  in  melilite,  234 

sieb,  197 

sieve,  197 

Stubachite,  276,  313 
Subnormal  interference  colors,  78 


Sulphur,  255 

chemical  test  for,  171 
Sulphuric  acid,  separation  by,  151 
Summary  of  petrographic  methods, 

135 

Sunstone,  208,  328 
Supernormal  interference  colors,  78 
Surface  color,  60 
Surirella  gemma,  26 

Table  of  interference  colors,  75 

Tachylite,  221 

Talc,  316 

Tantalum,  chemical  test  for,  170 

Tetrabrom  acetylene,  156 

Thallium  mercurous  nitrate,  159 

Thallium  silver  nitrate,  159 

Thickness,  measurement  of,  45 

Thin  sections,  preparation  of,  144 

Thomsonite,  341 

Thorium,  chemical  test  for,  168 

Thoulet  solution,  157 

Thulite,  261 

Thuringite,  312 

Tin,  chemical  test  for,  170 

Titanite,  255 

Titanium,  chemical  test  for,  170 

Titanium  magnetite,  206 

Titanium  olivine,  276 

Tomlinson,  W.  H.,  144 

Topaz,  301 

Topazolite,  217 

Total  reflection,  31 

critical  angle  of ,  31 
Total  refractometer,  Abbe,  39 
Total  reflectometer,  Wallerant,  40 
Tourmaline,  234 

hemimorphic  development  of, 
235 

suns,  235 

tongs,  10 
Tremolite,  289 
Trichite,  44,  189 
Trichroic,  62 
Tridymite,  249 
Trillings,  121 

Tungsten,  chemical  test  for,  168 
Turner's  test,  169 
Twins,  120 


396 


INDEX 


Twins,  cruciform,  184 

juxtaposition,  184 

penetration,  184 

uniaxial  with  inclined  axes,  120 
Twin,  compensator,  92 

lamination,  122 

polarizer,  73 
Twinning,  184 

lamination  in  leucite,  220 

repeated,  185 

Ultramicroscopy,  7 

Ultraviolet  rays,  7 

Undulatory  extinction,  191 

Uniaxial  crystals,  55 

Uniaxial  interference  figure,  100 

Uniaxial    interference    figure    with 

parallel  nicols,  100 
Uniaxial    minerals    in    convergent 

light,  99 
Uralite,  283 
Uralite  gabbro,  283 
Uralite  porphyrite,  283 
Uralitization,  283 
Uwarowite,  217 

Vanadium,  chemical  test  for,  170 

Velocity  of  light,  8 

Verant  lens,  3 

Vertical  illuminator,  49 

Vesuvianite,  233 

Vibration  directions,  determination 

of  position  of,  67 
Vibration  of  polarized  light,  8 
Villarsite,  312 
Violaite,  285 


Violet  I,  test  plate,  72 
Viridite,  311 
Virtual  image,  1 

Wache,  283 

Wagnerite,  318 

Wallerant  total  reflectometer,  40 

Water,  test  for,  172 

Wavellite,  320 

Wave  surface  of  uniaxial  crystals, 

56 

Wavy,  extinction  191 
Westphal  balance,  177 
White  of  the  higher  order,  77 
Woehlerite,  257 
Wollastonite,  299 
Wollastonite  group,  299 
Wright  wedge,  93 
Wurtzite,  230 

Xenimorphic,  181 
Xenotime,  231 

Yttrium,  chemical  test  for,  168 

Zeolites,  biaxial,  341 

Zinc  chloride,  159 

Zinc  spinel,  218 

Zircon,  230 

Zircon  syenite,  230 

Zirconium,  chemical  test  for,  170 

Zirkelite,  213 

Zoisitea,  260 

Zoisite  /?,  260 

Zonal  development,  188 

Zonal  structure  in  plagioclase,  334 


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